Generate a marker token when entering or leaving a submodule when building a

Generate a marker token when entering or leaving a submodule when building a
module. Use the marker to diagnose cases where we try to transition between
submodules when not at the top level (most likely because a closing brace was
missing at the end of a header file, but is also possible if submodule headers
attempt to do something fundamentally non-modular, like our .def files).

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@195543 91177308-0d34-0410-b5e6-96231b3b80d8