[modules] Add a flag to disable the feature that permits conflicting...

[modules] Add a flag to disable the feature that permits conflicting redefinitions of internal-linkage symbols that are not visible.

Such conflicts are an accident waiting to happen, and this feature conflicts
with the desire to include existing headers into multiple modules and merge the
results. (In an ideal world, it should not be possible to export internal
linkage symbols from a module, but sadly the glibc and libstdc++ headers
provide 'static inline' functions in a few cases.)

llvm-svn: 240335