[Modules] Fix a sneaky bug in r233249 where we would look for implicit

constructors in the current lexical context even though name lookup
found them via some other context merged into the redecl chain.

This can only happen for implicit constructors which can only have the
name of the type of the current context, so we can fix this by simply
*always* merging those names first. This also has the advantage of
removing the walk of the current lexical context from the common case
when this is the only constructor name we need to deal with (implicit or
otherwise).

I've enhanced the tests to cover this case (and uncovered an unrelated
bug which I fixed in r233325).

llvm-svn: 233327