[Sema] Avoid "case value not in enumerated type" warning for C++11 opaque enums

This commit ensures that the switch warning "case value not in enumerated type"
isn't shown for opaque enums. We don't know the actual list of values in opaque
enums, so that warning is incorrect.

rdar://29230764

Differential Revision: https://reviews.llvm.org/D27299

llvm-svn: 289055