[modules] Use DeclContext::equals rather than == on DeclContext* when
determining whether a declaration is out of line, instead of assuming
that the semantic and lexical DeclContext will be the same declaration
whenever they're the same entity.

This fixes behavior of declarations within merged classes and enums.


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@217008 91177308-0d34-0410-b5e6-96231b3b80d8