[CIR][Test] Add divergence tests for vtable emission issues

This commit adds three tests documenting divergences between CIR and CodeGen
in vtable emission. All tests verify that CIR should emit the same attributes
as CodeGen but currently fails to do so.

**Tests Added:**
1. vtable-constant-divergence.cpp - Documents missing 'constant' attribute
2. vtable-unnamed-addr-divergence.cpp - Documents missing 'unnamed_addr' attribute
3. vtable-comdat-divergence.cpp - Documents missing 'comdat' attribute

**Issue:**
CIR emits vtables with 'global' linkage:
  @_ZTV4Base = linkonce_odr global { [3 x ptr] } ...

CodeGen correctly emits them as 'constant' with additional attributes:
  @_ZTV4Base = linkonce_odr unnamed_addr constant { [3 x ptr] } ... comdat

**Root Cause:**
In CIRGenModule.cpp:4081, createOrReplaceCXXRuntimeVariable() calls
createGlobalOp() without passing isConstant=true parameter. Additionally,
the unnamed_addr and comdat attributes are not being set.

**Impact:**
1. Vtables placed in .data instead of .rodata section
2. Security vulnerability - writable vtables enable exploits
3. Missed optimization opportunities from unnamed_addr merging
4. Incorrect linker behavior without comdat groups
5. Diverges from CodeGen without justification

All tests are marked XFAIL until the fixes are implemented. Each test uses
FileCheck to verify both CIR and CodeGen outputs, with the CIR check failing
due to the missing attributes.

Test Plan:
Build and verify tests fail as expected (documenting divergence):
  bin/llvm-lit -v ../../clang/test/CIR/CodeGen/vtable-*-divergence.cpp

Author: lanza

clang/test/CIR/CodeGen/vtable-comdat-divergence.cpp

@@ -0,0 +1,36 @@
+// RUN: %clang_cc1 -triple x86_64-unknown-linux-gnu -fclangir -emit-llvm %s -o %t-cir.ll
+// RUN: %clang_cc1 -triple x86_64-unknown-linux-gnu -emit-llvm %s -o %t-codegen.ll
+// RUN: FileCheck --check-prefix=CIR --input-file=%t-cir.ll %s
+// RUN: FileCheck --check-prefix=CODEGEN --input-file=%t-codegen.ll %s
+
+// XFAIL: *
+
+// This test documents a divergence between CIR and CodeGen:
+// CIR does not emit 'comdat' attribute on vtables.
+// This is a bug that needs to be fixed.
+//
+// Expected (CodeGen):
+//   @_ZTV4Base = linkonce_odr unnamed_addr constant { [3 x ptr] } ... comdat
+//
+// Actual (CIR):
+//   @_ZTV4Base = linkonce_odr global { [3 x ptr] } ...
+//
+// The vtable should be marked with 'comdat' because:
+// 1. Enables proper handling of linkonce_odr definitions across translation units
+// 2. Ensures the linker can safely discard duplicate vtable definitions
+// 3. Required for correct C++ semantics with inline/template classes
+// 4. CodeGen has always emitted them with this attribute
+
+class Base {
+public:
+  virtual void foo() {}
+};
+
+void test() {
+  Base b;
+  b.foo();
+}
+
+// Both should emit comdat attribute
+// CIR: @_ZTV4Base = {{.*}}, comdat
+// CODEGEN: @_ZTV4Base = {{.*}}, comdat

clang/test/CIR/CodeGen/vtable-constant-divergence.cpp

@@ -0,0 +1,36 @@
+// RUN: %clang_cc1 -triple x86_64-unknown-linux-gnu -fclangir -emit-llvm %s -o %t-cir.ll
+// RUN: %clang_cc1 -triple x86_64-unknown-linux-gnu -emit-llvm %s -o %t-codegen.ll
+// RUN: FileCheck --check-prefix=CIR --input-file=%t-cir.ll %s
+// RUN: FileCheck --check-prefix=CODEGEN --input-file=%t-codegen.ll %s
+
+// XFAIL: *
+
+// This test documents a divergence between CIR and CodeGen:
+// CIR emits vtables as 'global' (mutable) instead of 'constant' (immutable).
+// This is a bug that needs to be fixed.
+//
+// Expected (CodeGen):
+//   @_ZTV4Base = linkonce_odr unnamed_addr constant { [3 x ptr] } ...
+//
+// Actual (CIR):
+//   @_ZTV4Base = linkonce_odr global { [3 x ptr] } ...
+//
+// The vtable should be marked as 'constant' because:
+// 1. Vtables are never modified at runtime
+// 2. They should be placed in .rodata (read-only data section)
+// 3. Writable vtables are a security vulnerability
+// 4. CodeGen has always emitted them as constant
+
+class Base {
+public:
+  virtual void foo() {}
+};
+
+void test() {
+  Base b;
+  b.foo();
+}
+
+// Both should emit vtable as constant
+// CIR: @_ZTV4Base = linkonce_odr unnamed_addr constant
+// CODEGEN: @_ZTV4Base = linkonce_odr unnamed_addr constant

clang/test/CIR/CodeGen/vtable-unnamed-addr-divergence.cpp

@@ -0,0 +1,36 @@
+// RUN: %clang_cc1 -triple x86_64-unknown-linux-gnu -fclangir -emit-llvm %s -o %t-cir.ll
+// RUN: %clang_cc1 -triple x86_64-unknown-linux-gnu -emit-llvm %s -o %t-codegen.ll
+// RUN: FileCheck --check-prefix=CIR --input-file=%t-cir.ll %s
+// RUN: FileCheck --check-prefix=CODEGEN --input-file=%t-codegen.ll %s
+
+// XFAIL: *
+
+// This test documents a divergence between CIR and CodeGen:
+// CIR does not emit 'unnamed_addr' attribute on vtables.
+// This is a bug that needs to be fixed.
+//
+// Expected (CodeGen):
+//   @_ZTV4Base = linkonce_odr unnamed_addr constant { [3 x ptr] } ...
+//
+// Actual (CIR):
+//   @_ZTV4Base = linkonce_odr global { [3 x ptr] } ...
+//
+// The vtable should be marked as 'unnamed_addr' because:
+// 1. The address of a vtable is never taken or used for identity comparison
+// 2. This allows the linker to merge duplicate vtables across translation units
+// 3. Reduces binary size and improves performance
+// 4. CodeGen has always emitted them with this attribute
+
+class Base {
+public:
+  virtual void foo() {}
+};
+
+void test() {
+  Base b;
+  b.foo();
+}
+
+// Both should emit unnamed_addr attribute
+// CIR: @_ZTV4Base = linkonce_odr unnamed_addr
+// CODEGEN: @_ZTV4Base = linkonce_odr unnamed_addr