[clang] [NFC] use appropriate castas (PR #95272)

[Simon Pilgrim via cfe-commits]

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@@ -2834,7 +2834,7 @@ ABIArgInfo
 X86_64ABIInfo::classifyRegCallStructTypeImpl(QualType Ty, unsigned &NeededInt,
                                              unsigned &NeededSSE,
                                              unsigned &MaxVectorWidth) const {
-  auto RT = Ty->getAs<RecordType>();
+  auto RT = Ty->castAs<RecordType>();
   assert(RT && "classifyRegCallStructType only valid with struct types");
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RKSimon wrote:

castAs has an internal assert so drop this one?

https://github.com/llvm/llvm-project/pull/95272