Consider:
1) Generate PCH with -fmodules and -fmodule-map-file
2) Use PCH with -fmodules and the same -fmodule-map-file
If we don't load -fmodule-map-file content before including PCHs,
the modules that are dependencies in PCHs cannot get loaded,
since there's no matching module map file when reading back the AST.
rdar://problem/40852867
Differential Revision: https://reviews.llvm.org/D48685
llvm-svn: 337447
Bruno Cardoso Lopes