[clang] [CodeGen] Revamp counted_by calculations (PR #70606)

[Nick Desaulniers via cfe-commits]

================
@@ -966,9 +962,68 @@ static llvm::Value *getArrayIndexingBound(CodeGenFunction &CGF,
   return nullptr;
 }
 
-FieldDecl *CodeGenFunction::FindCountedByField(
-    const Expr *Base,
-    LangOptions::StrictFlexArraysLevelKind StrictFlexArraysLevel) {
+Expr *CodeGenFunction::BuildCountedByFieldExpr(Expr *Base,
+                                               const ValueDecl *CountedByVD) {
+  // Find the outer struct expr (i.e. p in p->a.b.c.d).
+  Base = Base->IgnoreImpCasts();
+  Base = Base->IgnoreParenNoopCasts(getContext());
+
+  // Work our way up the expression until we reach the DeclRefExpr.
+  while (!isa<DeclRefExpr>(Base))
+    if (auto *ME = dyn_cast<MemberExpr>(Base->IgnoreImpCasts())) {
+      Base = ME->getBase()->IgnoreImpCasts();
+      Base = Base->IgnoreParenNoopCasts(getContext());
+    }
----------------
nickdesaulniers wrote:

That's ignoring a lot of casts! Could this perhaps be:

```
if (auto *ME = dyn_cast<MemberExpr>(Base))
  Base = ME->getBase()->IgnoreImpCasts()->IgnoreParentNoopCasts(getContext);
```

The call to `IgnoreImpCasts` in the `if` seems handled before the loop, and in the loop body?

https://github.com/llvm/llvm-project/pull/70606