[KeyInstr][Clang] Switch stmt 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/CGStmt.cpp

@@ -2276,6 +2276,8 @@ void CodeGenFunction::EmitSwitchStmt(const SwitchStmt &S) {
   // failure.
   llvm::BasicBlock *DefaultBlock = createBasicBlock("sw.default");
   SwitchInsn = Builder.CreateSwitch(CondV, DefaultBlock);
+  addInstToNewSourceAtom(SwitchInsn, CondV);
+
   if (HLSLControlFlowAttr != HLSLControlFlowHintAttr::SpellingNotCalculated) {
     llvm::MDBuilder MDHelper(CGM.getLLVMContext());
     llvm::ConstantInt *BranchHintConstant =

clang/test/DebugInfo/KeyInstructions/switch.c

@@ -0,0 +1,51 @@
+// RUN: %clang -gkey-instructions -x c++ -std=c++17 %s -gmlt -S -emit-llvm -o - \
+// RUN: | FileCheck %s --implicit-check-not atomGroup --implicit-check-not atomRank --check-prefixes=CHECK,CHECK-CXX
+
+// RUN: %clang -gkey-instructions -x c %s -gmlt -S -emit-llvm -o -  \
+// RUN: | FileCheck %s --implicit-check-not atomGroup --implicit-check-not atomRank
+
+int g;
+void a(int A, int B) {
+// CHECK: entry:
+// The load gets associated with the branch rather than the store.
+// FIXME: Is that the best thing to do?
+// CHECK: %0 = load i32, ptr %A.addr{{.*}}, !dbg [[G2R2:!.*]]
+// CHECK: store i32 %0, ptr @g{{.*}}, !dbg [[G1R1:!.*]]
+// CHECK: switch i32 %0, label %{{.*}} [
+// CHECK:   i32 0, label %sw.bb
+// CHECK:   i32 1, label %sw.bb1
+// CHECK: ], !dbg [[G2R1:!.*]]
+    switch ((g = A)) {
+    case 0: break;
+    case 1: {
+// CHECK: sw.bb1:
+// CHECK: %1 = load i32, ptr %B.addr{{.*}}, !dbg [[G3R2:!.*]]
+// CHECK: switch i32 %1, label %{{.*}} [
+// CHECK:   i32 0, label %sw.bb2
+// CHECK: ], !dbg [[G3R1:!.*]]
+    switch ((B)) {
+        case 0: {
+// Test that assignments in constant-folded switches don't go missing.
+// CHECK-CXX: sw.bb2:
+// CHECK-CXX: store i32 1, ptr %C{{.*}}, !dbg [[G4R1:!.*]]
+#ifdef __cplusplus
+            switch (const int C = 1; C) {
+                case 0: break;
+                case 1: break;
+                default: break;
+            }
+#endif
+        } break;
+        default: break;
+    }
+    } break;
+    default: break;
+    }
+}
+
+// CHECK: [[G2R2]] = !DILocation({{.*}}, atomGroup: 2, atomRank: 2)
+// 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)
+// CHECK-CXX: [[G4R1]] = !DILocation({{.*}}, atomGroup: 4, atomRank: 1)