Don't overwrite the ExternalSemaSource in the modules case. (#538)
Don't overwrite the ExternalSemaSource in the modules case. (#538)

The modules implementation provides it's own sema source and
overwriting this confuses clang when we have -fmodules enabled.

karies/cling,root-mirror/cling,root-mirror/cling,karies/cling,karies/cling,marsupial/cling,karies/cling,marsupial/cling,marsupial/cling,root-mirror/cling,root-mirror/cling,marsupial/cling,marsupial/cling,marsupial/cling,root-mirror/cling,karies/cling,root-mirror/cling,karies/cling