[clang] [ObjC] Fix Assertion failure when merging declarations with different lifetime qualifiers (PR #203272)

[Hendrik Hübner via cfe-commits]

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@@ -12339,7 +12339,7 @@ QualType ASTContext::mergeObjCGCQualifiers(QualType LHS, QualType RHS) {
       // id foo(); ... __strong id foo(); or: __strong id foo(); ... id foo();
       // In either case, use OldReturnType to build the new function type.
       const auto *F = LHS->castAs<FunctionType>();
-      if (const auto *FPT = cast<FunctionProtoType>(F)) {
+      if (const auto *FPT = cast_or_null<FunctionProtoType>(F)) {
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HendrikHuebner wrote:

Yeah I guess it should be a `dyn_cast`.

https://github.com/llvm/llvm-project/pull/203272