[clang] Fix a crash on CheckArgAlignment.

We might encounter an undeduced type before calling getTypeAlignInChars.

NOTE: this retrieves the fix from
8f80c66b, which was removed in Adam's
followup fix fbfcfdbf. We originally
thought the crash was caused by recovery-ast, but it turns out it can
occur for other cases, e.g. typo-correction.

Differential Revision: https://reviews.llvm.org/D102750