PR19215: When writing/reading a PCH that imported a module, store the location
at which that PCH imported each visible submodule of the module. Such locations
are needed when synthesizing macro directives resulting from the import.


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@204417 91177308-0d34-0410-b5e6-96231b3b80d8