[KeyInstr][Clang] Aggregate init + copy

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/CGExprAgg.cpp

@@ -2393,7 +2393,8 @@ void CodeGenFunction::EmitAggregateCopy(LValue Dest, LValue Src, QualType Ty,
     }
   }
 
-  auto Inst = Builder.CreateMemCpy(DestPtr, SrcPtr, SizeVal, isVolatile);
+  auto *Inst = Builder.CreateMemCpy(DestPtr, SrcPtr, SizeVal, isVolatile);
+  addInstToCurrentSourceAtom(Inst, nullptr);
 
   // Determine the metadata to describe the position of any padding in this
   // memcpy, as well as the TBAA tags for the members of the struct, in case

clang/test/DebugInfo/KeyInstructions/agg.c

@@ -0,0 +1,12 @@
+
+// RUN: %clang -gkey-instructions %s -gmlt -S -emit-llvm -o - \
+// RUN: | FileCheck %s --implicit-check-not atomGroup --implicit-check-not atomRank
+
+typedef struct { int a, b, c; } Struct;
+void fun(Struct a) {
+// CHECK: call void @llvm.memcpy{{.*}}, !dbg  [[G1R1:!.*]]
+  Struct b = a;
+}
+
+// CHECK: [[G1R1]] = !DILocation({{.*}}, atomGroup: 1, atomRank: 1)
+