[Sema] Avoid using a null type pointer (fixes PR32750)

isMicrosoftMissingTypename() uses a Type pointer without first checking
that it's non-null. PR32750 reports a case where the pointer is in fact
null. This patch adds in a defensive check and a regression test.

Differential Revision: https://reviews.llvm.org/D32519

llvm-svn: 301420