[KeyInstr][Clang] Assignment atom group

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

clang/lib/CodeGen/CGExpr.cpp

@@ -5814,6 +5814,15 @@ LValue CodeGenFunction::EmitBinaryOperatorLValue(const BinaryOperator *E) {
 
   assert(E->getOpcode() == BO_Assign && "unexpected binary l-value");
 
+  // This covers both LHS and RHS expressions, though nested RHS
+  // expressions may get subsequently separately grouped.
+  // FIXME(OCH): Not clear yet if we've got fine enough control
+  // to pick and choose when we need to. Currently looks ok:
+  // a = b = c  -> Two atoms.
+  // x = new(1) -> One atom (for both addr store and value store).
+  // Complex and agg assignment -> One atom.
+  ApplyAtomGroup Grp(getDebugInfo());
+
   // Note that in all of these cases, __block variables need the RHS
   // evaluated first just in case the variable gets moved by the RHS.
 

clang/test/KeyInstructions/assign.cpp

@@ -0,0 +1,9 @@
+// RUN: %clang -gkey-instructions %s -gmlt -gcolumn-info -S -emit-llvm -o - -Wno-unused-variable \
+// RUN: | FileCheck %s --implicit-check-not atomGroup --implicit-check-not atomRank
+
+unsigned long long g;
+void fun() { g = 0; }
+
+// CHECK: store i64 0, ptr @g{{.*}}, !dbg [[G1R1:!.*]]
+
+// CHECK: [[G1R1]] = !DILocation({{.*}}, atomGroup: 1, atomRank: 1)