Change enumerator default linkage type for C

Redeclaration lookup should never find hidden enumerators in C, because
they do not have linkage (C11 6.2.2/6)

The linkage of an enumerator should be VisibleNoLinkage, and
isHiddenDeclarationVisible should be checking hasExternalFormalLinkage.

This is was reviewed as part of D31778, but splitted into a different
commit for clarity.

rdar://problem/31909368

llvm-svn: 306917