DebugInfo: Remove LexicalBlockFile scope/context distinction

Two things here:

 1. I read `getScope()` and `getContext()` backwards in r233640.  There
    was no need for `getScopeOfScope()`.  Obviously not enough test
    coverage here (as I said in that commit, I'm going to come back to
    that), but anyway I'm reverting to the behaviour before r233640.
 2. The callers that use `DILexicalBlockFile::getContext()` don't seem
    to care about the difference.  Just have it redirect to `getScope()`
    so I can't get confused again.

llvm-svn: 233650