[clang] [Clang] Generate the GEP instead of adding AST nodes (PR #73730)

[Nick Desaulniers via cfe-commits]

================
@@ -4022,8 +4168,36 @@ LValue CodeGenFunction::EmitArraySubscriptExpr(const ArraySubscriptExpr *E,
       ArrayLV = EmitArraySubscriptExpr(ASE, /*Accessed*/ true);
     else
       ArrayLV = EmitLValue(Array);
+
     auto *Idx = EmitIdxAfterBase(/*Promote*/true);
 
+    if (SanOpts.has(SanitizerKind::ArrayBounds)) {
+      const LangOptions::StrictFlexArraysLevelKind StrictFlexArraysLevel =
+          getLangOpts().getStrictFlexArraysLevel();
+
+      if (const auto *ME = dyn_cast<MemberExpr>(Array);
+          ME &&
+          ME->isFlexibleArrayMemberLike(getContext(), StrictFlexArraysLevel) &&
+          ME->getMemberDecl()->hasAttr<CountedByAttr>()) {
+        RecordDecl *RD = ME->getMemberDecl()
+                             ->getDeclContext()
+                             ->getOuterLexicalRecordContext();
+        Expr *StructBase =
+            StructAccessBase(RD).Visit(const_cast<MemberExpr *>(ME));
+
+        if (StructBase && StructBase->getType()->isPointerType()) {
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nickdesaulniers wrote:

`ME` is declared inside the `if` but `StructBase` is not.  I don't care particularly which pattern you use, but let's use one style consistently? (either move `ME` out or `StructBase` in?)

if you make the `if` with `ME` just

```
if (const auto *ME = ...) {
```
then you could sink getting the flex array level to within the `if`.

https://github.com/llvm/llvm-project/pull/73730