[CodeGen,test] Test llvm-libc style alias attribute with UsingShadowDecl

The pattern is quite involved and deserves a specific codegen test.
This test would catch the bug in the first attempt of #87130

Author: MaskRay

clang/test/CodeGen/alias.cpp

@@ -1,27 +1,42 @@
-// RUN: %clang_cc1 -triple x86_64-linux -verify -emit-llvm-only %s
-// RUN: not %clang_cc1 -triple x86_64-linux -emit-llvm-only -fdiagnostics-parseable-fixits %s 2>&1 | FileCheck %s
+// RUN: %clang_cc1 -triple x86_64-linux -verify -emit-llvm-only -DERR %s
+// RUN: not %clang_cc1 -triple x86_64-linux -emit-llvm-only -fdiagnostics-parseable-fixits -DERR %s 2>&1 | FileCheck %s --check-prefix=FIXIT
+// RUN: %clang_cc1 -triple x86_64-linux -emit-llvm %s -o - | FileCheck %s
 
+#ifdef ERR
 void *f1_ifunc(void) { return nullptr; }
 void f1(void) __attribute__((alias("f1_ifunc")));
 // expected-error@-1 {{alias must point to a defined variable or function}}
 // expected-note@-2 {{must refer to its mangled name}}
 // expected-note@-3 {{function by that name is mangled as}}
-// CHECK: fix-it:"{{.*}}":{[[@LINE-4]]:30-[[@LINE-4]]:47}:"alias(\"_Z8f1_ifuncv\")"
+// FIXIT: fix-it:"{{.*}}":{[[@LINE-4]]:30-[[@LINE-4]]:47}:"alias(\"_Z8f1_ifuncv\")"
 
 void *f6_resolver_resolver(void) { return 0; }
 void *f6_resolver(void) __attribute__((alias("f6_resolver_resolver")));
 // expected-error@-1 {{alias must point to a defined variable or function}}
 // expected-note@-2 {{must refer to its mangled name}}
 // expected-note@-3 {{function by that name is mangled as}}
-// CHECK: fix-it:"{{.*}}":{[[@LINE-4]]:40-[[@LINE-4]]:69}:"alias(\"_Z20f6_resolver_resolverv\")"
+// FIXIT: fix-it:"{{.*}}":{[[@LINE-4]]:40-[[@LINE-4]]:69}:"alias(\"_Z20f6_resolver_resolverv\")"
 void f6(void) __attribute__((alias("f6_resolver")));
 // expected-error@-1 {{alias must point to a defined variable or function}}
 // expected-note@-2 {{must refer to its mangled name}}
 // expected-note@-3 {{function by that name is mangled as}}
-// CHECK: fix-it:"{{.*}}":{[[@LINE-4]]:30-[[@LINE-4]]:50}:"alias(\"_Z11f6_resolverv\")"
+// FIXIT: fix-it:"{{.*}}":{[[@LINE-4]]:30-[[@LINE-4]]:50}:"alias(\"_Z11f6_resolverv\")"
 
 __attribute__((unused, alias("resolver"), deprecated("hahahaha, isn't C great?")))
 void func();
 // expected-error@-2 {{alias must point to a defined variable or function}}
 // expected-note@-3 {{must refer to its mangled name}}
+#endif
 
+// CHECK: @_ZN4libc4log2Ed ={{.*}} alias double (double), ptr @log2
+// CHECK: define{{.*}} @log2(
+namespace libc { double log2(double x); }
+extern "C" double log2(double);
+namespace std { using ::log2; }
+using std::log2;
+
+namespace libc {
+decltype(libc::log2) __log2_impl__ __asm__("log2");
+decltype(libc::log2) log2 [[gnu::alias("log2")]];
+double __log2_impl__(double x) { return x; }
+}