[KeyInstr][Clang] Agg 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/CGDecl.cpp

@@ -1270,6 +1270,8 @@ void CodeGenFunction::emitStoresForConstant(const VarDecl &D, Address Loc,
                            createUnnamedGlobalForMemcpyFrom(
                                CGM, D, Builder, constant, Loc.getAlignment()),
                            SizeVal, isVolatile);
+  addInstToCurrentSourceAtom(I, nullptr);
+
   if (IsAutoInit)
     I->addAnnotationMetadata("auto-init");
 }

clang/test/DebugInfo/KeyInstructions/init-agg.cpp

@@ -0,0 +1,22 @@
+
+// RUN: %clang -gkey-instructions %s -gmlt -S -emit-llvm -o - \
+// RUN: | FileCheck %s --implicit-check-not atomGroup --implicit-check-not atomRank
+
+// The implicit-check-not is important; we don't want the GEPs created for the
+// store locations to be included in the atom group.
+
+int g;
+void a() {
+// CHECK: _Z1av()
+// CHECK: call void @llvm.memcpy{{.*}}, !dbg [[G1R1:!.*]]
+    int A[] = { 1, 2, 3 };
+// CHECK: store i32 1, ptr %{{.*}}, !dbg [[G2R1:!.*]]
+// CHECK: store i32 2, ptr %{{.*}}, !dbg [[G2R1]]
+// CHECK: %0 = load i32, ptr @g{{.*}}, !dbg [[G2R2:!.*]]
+// CHECK: store i32 %0, ptr %{{.*}}, !dbg [[G2R1]]
+    int B[] = { 1, 2, g };
+}
+
+// CHECK: [[G1R1]] = !DILocation({{.*}}, atomGroup: 1, atomRank: 1)
+// CHECK: [[G2R1]] = !DILocation({{.*}}, atomGroup: 2, atomRank: 1)
+// CHECK: [[G2R2]] = !DILocation({{.*}}, atomGroup: 2, atomRank: 2)