[modules] Generalize the workaround for multiple ambiguous definitions of

internal linkage entities in different modules from r250884 to apply to all
names, not just function names.

This is really awkward: we don't want to merge internal-linkage symbols from
separate modules, because they might not actually be defining the same entity.
But we don't want to reject programs that use such an ambiguous symbol if those
internal-linkage symbols are in fact equivalent. For now, we're resolving the
ambiguity by picking one of the equivalent definitions as an extension.

llvm-svn: 252063