[PCH+Modules] Load -fmodule-map-file content before including PCHs

[PCH+Modules] Load -fmodule-map-file content before including PCHs

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

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@337447 91177308-0d34-0410-b5e6-96231b3b80d8