[KeyInstr][Clang] Catch variable init atom

This patch is part of a stack that teaches Clang to generate Key Instructions
metadata for C and C++.

The Key Instructions project is introduced, including a "quick summary" section
at the top which adds context for this PR, here:
https://discourse.llvm.org/t/rfc-improving-is-stmt-placement-for-better-interactive-debugging/82668

The feature is only functional in LLVM if LLVM is built with CMake flag
LLVM_EXPERIMENTAL_KEY_INSTRUCTIONs. Eventually that flag will be removed.

The Clang-side work is demoed here:
#130943

Author: OCHyams

clang/lib/CodeGen/ItaniumCXXABI.cpp

@@ -5051,6 +5051,7 @@ void ItaniumCXXABI::emitBeginCatch(CodeGenFunction &CGF,
 
   // Emit the local.
   CodeGenFunction::AutoVarEmission var = CGF.EmitAutoVarAlloca(*CatchParam);
+  ApplyAtomGroup Grp(CGF.getDebugInfo());
   InitCatchParam(CGF, *CatchParam, var.getObjectAddress(CGF), S->getBeginLoc());
   CGF.EmitAutoVarCleanups(var);
 }

clang/test/KeyInstructions/try-catch.cpp

@@ -0,0 +1,20 @@
+// RUN: %clang -gkey-instructions %s -gmlt -S -emit-llvm -o - -fexceptions \
+// RUN: | FileCheck %s
+
+void except() {
+  // FIXME(OCH): Should `store i32 32, ptr %exception` be key?
+  throw 32;
+}
+
+void attempt() {
+  try { except(); }
+// CHECK: catch:
+// CHECK: %4 = call ptr @__cxa_begin_catch(ptr %exn)
+// CHECK: %5 = load i32{{.*}}, !dbg [[G1R2:!.*]]
+// CHECK: store i32 %5, ptr %e{{.*}}, !dbg [[G1R1:!.*]]
+// CHECK: call void @__cxa_end_catch()
+  catch (int e) { }
+}
+
+// CHECK: [[G1R2]] = !DILocation({{.*}}, atomGroup: 1, atomRank: 2)
+// CHECK: [[G1R1]] = !DILocation({{.*}}, atomGroup: 1, atomRank: 1)