[KeyInstr][Clang] Assign vector element 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

@@ -2507,8 +2507,9 @@ void CodeGenFunction::EmitStoreThroughLValue(RValue Src, LValue Dst,
         Vec = Builder.CreateBitCast(Vec, IRStoreTy);
       }
 
-      Builder.CreateStore(Vec, Dst.getVectorAddress(),
-                          Dst.isVolatileQualified());
+      auto *I = Builder.CreateStore(Vec, Dst.getVectorAddress(),
+                                    Dst.isVolatileQualified());
+      addInstToCurrentSourceAtom(I, Vec);
       return;
     }
 

clang/test/DebugInfo/KeyInstructions/agg.c

@@ -4,14 +4,22 @@
 // RUN: %clang_cc1 -gkey-instructions -x c %s -debug-info-kind=line-tables-only -emit-llvm -o - \
 // RUN: | FileCheck %s --implicit-check-not atomGroup --implicit-check-not atomRank
 
+__attribute__((ext_vector_type(1))) char c;
 typedef struct { int a, b, c; } Struct;
 void fun(Struct a) {
 // CHECK: call void @llvm.memcpy{{.*}}, !dbg [[G1R1:!.*]]
   Struct b = a;
 
 // CHECK: call void @llvm.memcpy{{.*}}, !dbg [[G2R1:!.*]]
   b = a;
+
+// CHECK: %2 = load <1 x i8>, ptr @c
+// CHECK: %vecins = insertelement <1 x i8> %2, i8 0, i32 0, !dbg [[G3R2:!.*]]
+// CHECK: store <1 x i8> %vecins, ptr @c{{.*}}, !dbg [[G3R1:!.*]]
+  c[0] = 0;
 }
 
 // CHECK: [[G1R1]] = !DILocation({{.*}}, atomGroup: 1, atomRank: 1)
 // CHECK: [[G2R1]] = !DILocation({{.*}}, atomGroup: 2, atomRank: 1)
+// CHECK: [[G3R2]] = !DILocation({{.*}}, atomGroup: 3, atomRank: 2)
+// CHECK: [[G3R1]] = !DILocation({{.*}}, atomGroup: 3, atomRank: 1)