Sema: Prioritize @_exported imports over local non-public imports
Update the logic selecting the most restrictive import for a given
reference to account for @_exported imports from the local module. We
should always prioritize @_exported imports from the local module over
more restrictive same file imports. Only if an import from the same file
is also public we prefer it as it's more useful for diagnostics and
generally recommended to locally declare dependencies.
Also update the test that was meant to check this configuration to apply
two different variations, one for a module local @_exported and one
relying on the underlying clang module.
rdar://140924031
