[KeyIntsr][Clang] Builtins alloca auto-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/CGBuiltin.cpp

@@ -138,6 +138,7 @@ static void initializeAlloca(CodeGenFunction &CGF, AllocaInst *AI, Value *Size,
   if (CGF.CGM.stopAutoInit())
     return;
   auto *I = CGF.Builder.CreateMemSet(AI, Byte, Size, AlignmentInBytes);
+  CGF.addInstToCurrentSourceAtom(I, nullptr);
   I->addAnnotationMetadata("auto-init");
 }
 

clang/test/KeyInstructions/builtin.c

@@ -0,0 +1,24 @@
+
+// RUN: %clang -gkey-instructions -x c++ %s -gmlt -gno-column-info -S -emit-llvm -o - -ftrivial-auto-var-init=zero \
+// RUN: | FileCheck %s --implicit-check-not atomGroup --implicit-check-not atomRank
+
+// RUN: %clang -gkey-instructions -x c %s -gmlt -gno-column-info -S -emit-llvm -o - -ftrivial-auto-var-init=zero \
+// RUN: | FileCheck %s --implicit-check-not atomGroup --implicit-check-not atomRank
+
+void fun() {
+// CHECK: %a = alloca ptr, align 8
+// CHECK: %0 = alloca i8, i64 4{{.*}}, !dbg [[G1R2:!.*]]
+// CHECK: call void @llvm.memset{{.*}}, !dbg [[G1R1:!.*]], !annotation
+// CHECK: store ptr %0, ptr %a{{.*}}, !dbg [[G1R1:!.*]]
+    void *a = __builtin_alloca(4);
+
+// CHECK: %1 = alloca i8, i64 4{{.*}}, !dbg [[G2R2:!.*]]
+// CHECK: call void @llvm.memset{{.*}}, !dbg [[G2R1:!.*]], !annotation
+// CHECK: store ptr %1, ptr %b{{.*}}, !dbg [[G2R1:!.*]]
+    void *b = __builtin_alloca_with_align(4, 8);
+}
+
+// CHECK: [[G1R2]] = !DILocation({{.*}}, atomGroup: 1, atomRank: 2)
+// CHECK: [[G1R1]] = !DILocation({{.*}}, atomGroup: 1, atomRank: 1)
+// CHECK: [[G2R2]] = !DILocation({{.*}}, atomGroup: 2, atomRank: 2)
+// CHECK: [[G2R1]] = !DILocation({{.*}}, atomGroup: 2, atomRank: 1)