Fix 41 of the 61 tests which fail with modules enabled: we were computing and

caching the linkage for a declaration before we set up its redeclaration chain,
when determining whether a declaration could be a redeclaration of something
from an unimported submodule. We actually want to look at the declaration as if
it were not a redeclaration here, so compute the linkage but don't cache it.

llvm-svn: 178733