Merging r324514:

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r324514 | arphaman | 2018-02-07 21:45:39 +0100 (Wed, 07 Feb 2018) | 10 lines

[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

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git-svn-id: https://llvm.org/svn/llvm-project/cfe/branches/release_60@324602 91177308-0d34-0410-b5e6-96231b3b80d8