[Sema] Call CheckPlaceholderExpr to resolve typeof or decltype

placeholder expressions while an unevaluated context is still on the
expression evaluation context stack.

This prevents recordUseOfWeek from being called when a weak variable is
used as an operand of a decltype or a typeof expression and fixes
spurious -Warc-repeated-use-of-weak warnings.

rdar://problem/45742525

Differential Revision: https://reviews.llvm.org/D55662

llvm-svn: 350887