[modules] When a file is listed as a non-textual header in a module map, don't

enter it more than once, even if it doesn't have #include guards -- we already
know that it is intended to have the same effect every time it's included, and
it's already had that effect. This particularly helps with local submodule
visibility builds, where the include guard macro may not be visible in the
includer, but will become visible the moment we enter the included file.

llvm-svn: 237609