[KeyInstr][Clang] Scalar 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

@@ -1925,6 +1925,7 @@ void CodeGenFunction::EmitAutoVarInit(const AutoVarEmission &emission) {
 
   const VarDecl &D = *emission.Variable;
   auto DL = ApplyDebugLocation::CreateDefaultArtificial(*this, D.getLocation());
+  ApplyAtomGroup Grp(getDebugInfo());
   QualType type = D.getType();
 
   // If this local has an initializer, emit it now.

clang/lib/CodeGen/CGExpr.cpp

@@ -2222,6 +2222,8 @@ void CodeGenFunction::EmitStoreOfScalar(llvm::Value *Value, Address Addr,
   }
 
   llvm::StoreInst *Store = Builder.CreateStore(Value, Addr, Volatile);
+  addInstToCurrentSourceAtom(Store, Value);
+
   if (isNontemporal) {
     llvm::MDNode *Node =
         llvm::MDNode::get(Store->getContext(),

clang/test/DebugInfo/KeyInstructions/init-scalar.c

@@ -0,0 +1,19 @@
+// RUN: %clang -gkey-instructions -x c++ %s -gmlt -gcolumn-info -S -emit-llvm -o - \
+// RUN: | FileCheck %s --implicit-check-not atomGroup --implicit-check-not atomRank
+
+// RUN: %clang -gkey-instructions -x c %s -gmlt -gcolumn-info -S -emit-llvm -o - \
+// RUN: | FileCheck %s --implicit-check-not atomGroup --implicit-check-not atomRank
+
+void a() {
+// CHECK: store i32 0, ptr %A{{.*}}, !dbg [[G1R1:!.*]]
+    int A = 0;
+// CHECK: %add = add {{.*}}, !dbg [[G2R2:!.*]]
+// CHECK: store i32 %add, ptr %B, align 4, !dbg [[G2R1:!.*]]
+    int B = 2 * A + 1;
+// CHECK-TODO: ret{{.*}}, !dbg [[G3R1:!.*]]
+}
+
+// CHECK: [[G1R1]] = !DILocation({{.*}}, atomGroup: 1, atomRank: 1)
+// CHECK: [[G2R2]] = !DILocation({{.*}}, atomGroup: 2, atomRank: 2)
+// CHECK: [[G2R1]] = !DILocation({{.*}}, atomGroup: 2, atomRank: 1)
+// CHECK-TODO: [[G3R1]] = !DILocation({{.*}}, atomGroup: 3, atomRank: 1)