[KeyInstr][Clang] Bitfield 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/CGExpr.cpp

@@ -2614,7 +2614,8 @@ void CodeGenFunction::EmitStoreThroughBitfieldLValue(RValue Src, LValue Dst,
   }
 
   // Write the new value back out.
-  Builder.CreateStore(SrcVal, Ptr, Dst.isVolatileQualified());
+  auto *I = Builder.CreateStore(SrcVal, Ptr, Dst.isVolatileQualified());
+  addInstToCurrentSourceAtom(I, SrcVal);
 
   // Return the new value of the bit-field, if requested.
   if (Result) {

clang/test/KeyInstructions/bitfield.cpp

@@ -0,0 +1,13 @@
+// RUN: %clang -gkey-instructions %s -gmlt -gcolumn-info -S -emit-llvm -o - \
+// RUN: | FileCheck %s --implicit-check-not atomGroup --implicit-check-not atomRank
+
+struct S { int a:3; };
+
+void foo(int x, S s) {
+// CHECK: %bf.set = or i8 %bf.clear, %bf.value, !dbg [[G1R2:!.*]]
+// CHECK: store i8 %bf.set, ptr %s, align 4, !dbg [[G1R1:!.*]]
+  s.a = x;
+}
+
+// CHECK: [[G1R2]] = !DILocation({{.*}}, atomGroup: 1, atomRank: 2)
+// CHECK: [[G1R1]] = !DILocation({{.*}}, atomGroup: 1, atomRank: 1)