Update and fix LLVM_ENABLE_MODULES:

1) We need to add this flag prior to adding any other, in case the user has
specified a -fmodule-cache-path= flag in their custom CXXFLAGS. Such a flag
causes -Werror builds to fail, and thus all config checks fail, until we add
the corresponding -fmodules flag. The modules selfhost bot does this, for
instance.

2) Delete module maps that were putting .cpp files into modules.

3) Enable -fmodules-local-submodule-visibility, to get proper module
visibility rules applied across submodules of the same module. Disable
-fmodules for C builds, since that flag is not available there.

llvm-svn: 266502