Clang importer was picking the beginning of the main FileID as include location of its modules, which is the same location that the clang PCH mechanism is using as import location.
Make sure to use a different location otherwise we'll have problems with source order comparisons of clang source locations not being deterministic.
rdar://29137319
Previously headers from the bridging header and imported modules had the exact same top location so comparison with them
was not sound. This resulted in missing documentation comments from declarations and possible assertion hits while clang
was trying to determine the comment for a declaration, but I could not create a reduced test case that triggered an assertion hit.
rdar://34925466
