[KeyInstr][Clang] Coerced store atoms

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/CGCall.cpp

@@ -1432,7 +1432,8 @@ void CodeGenFunction::CreateCoercedStore(llvm::Value *Src, Address Dst,
       for (unsigned i = 0, e = STy->getNumElements(); i != e; ++i) {
         Address EltPtr = Builder.CreateStructGEP(Dst, i);
         llvm::Value *Elt = Builder.CreateExtractValue(Src, i);
-        Builder.CreateStore(Elt, EltPtr, DstIsVolatile);
+        auto *I = Builder.CreateStore(Elt, EltPtr, DstIsVolatile);
+        addInstToCurrentSourceAtom(I, Elt);
       }
     } else {
       Builder.CreateStore(Src, Dst.withElementType(SrcTy), DstIsVolatile);

clang/test/KeyInstructions/coerced.c

@@ -0,0 +1,24 @@
+// RUN: %clang -gkey-instructions -gno-column-info -x c++ %s -gmlt -S -emit-llvm -o - -target x86_64-unknown-linux \
+// RUN: | FileCheck %s --implicit-check-not atomGroup --implicit-check-not atomRank
+
+// RUN: %clang -gkey-instructions -gno-column-info -x c %s -gmlt -S -emit-llvm -o - -target x86_64-unknown-linux \
+// RUN: | FileCheck %s --implicit-check-not atomGroup --implicit-check-not atomRank
+
+typedef struct {
+  void* a;
+  void* b;
+} Struct;
+
+Struct get();
+void store() {
+  // CHECK: %1 = extractvalue { ptr, ptr } %call, 0, !dbg [[G1R2:!.*]]
+  // CHECK: store ptr %1, ptr {{.*}}, !dbg [[G1R1:!.*]]
+  // CHECK: %3 = extractvalue { ptr, ptr } %call, 1, !dbg [[G1R2]]
+  // CHECK: store ptr %3, ptr {{.*}}, !dbg [[G1R1:!.*]]
+  Struct s = get();
+  // CHECK: ret void, !dbg [[G2R1:!.*]]
+}
+
+// CHECK: [[G1R2]] = !DILocation({{.*}}, atomGroup: 1, atomRank: 2)
+// CHECK: [[G1R1]] = !DILocation({{.*}}, atomGroup: 1, atomRank: 1)
+// CHECK: [[G2R1]] = !DILocation({{.*}}, atomGroup: 2, atomRank: 1)