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