[modules] When considering merging a newly-declared typedef into an imported
one, perform the import if the types match even if the imported declaration is
hidden. Otherwise, NamedDecl::declarationReplaces will drop one of the name
lookup entries, making the typedef effectively inaccessible from one of the
modules that declared it.


git-svn-id: http://llvm.org/svn/llvm-project/cfe/trunk@215306 91177308-0d34-0410-b5e6-96231b3b80d8