[KeyInstr][Clang] Static 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/CGDecl.cpp

@@ -428,8 +428,10 @@ void CodeGenFunction::EmitStaticVarDecl(const VarDecl &D,
   bool isCudaSharedVar = getLangOpts().CUDA && getLangOpts().CUDAIsDevice &&
                          D.hasAttr<CUDASharedAttr>();
   // If this value has an initializer, emit it.
-  if (D.getInit() && !isCudaSharedVar)
+  if (D.getInit() && !isCudaSharedVar) {
+    ApplyAtomGroup Grp(getDebugInfo());
     var = AddInitializerToStaticVarDecl(D, var);
+  }
 
   var->setAlignment(alignment.getAsAlign());
 

clang/test/KeyInstructions/init-static.cpp

@@ -0,0 +1,12 @@
+// RUN: %clang -gkey-instructions %s -gmlt -gcolumn-info -S -emit-llvm -o -\
+// RUN: | FileCheck %s --implicit-check-not atomGroup --implicit-check-not atomRank
+
+void g(int *a) {
+    // CHECK: %2 = load ptr, ptr %a.addr{{.*}}, !dbg [[G1R2:!.*]]
+    // CHECK: store ptr %2, ptr @_ZZ1gPiE1b{{.*}}, !dbg [[G1R1:!.*]]
+    static int &b = *a;
+}
+
+// CHECK: [[G1R2]] = !DILocation({{.*}}, atomGroup: 1, atomRank: 2)
+// CHECK: [[G1R1]] = !DILocation({{.*}}, atomGroup: 1, atomRank: 1)
+