When generating debug symbols for private Clang types (which we started
importing recently), the compiler crashes due to an assertion failure
from ClangModuleUnit::getDiscriminatorForPrivateDecl(), which is called
by getFilePrivateScope().
This patch fixes the issue crash by not calling getFilePrivateScope()
for Clang types. A discriminator is usually needed to disambiguate
private Swift types declared in different files, but Clang types follow
different scoping conventions that make this discriminator unnecessary.
rdar://148481025
