[PR36008] Avoid -Wsign-compare warning for enum constants in

typeof expressions

This commit looks through typeof type at the original expression when diagnosing
-Wsign-compare to avoid an unfriendly diagnostic.

rdar://36588828

Differential Revision: https://reviews.llvm.org/D42561


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@324514 91177308-0d34-0410-b5e6-96231b3b80d8