[cxx-interop] Do not get private discriminator for private Clang types

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 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

Author: j-hui

lib/IRGen/IRGenDebugInfo.cpp

@@ -2578,7 +2578,7 @@ class IRGenDebugInfoImpl : public IRGenDebugInfo {
     // Scope outermost fileprivate decls in an inline private discriminator
     // namespace.
     if (auto *Decl = DbgTy.getDecl())
-      if (Decl->isOutermostPrivateOrFilePrivateScope())
+      if (!ClangDecl && Decl->isOutermostPrivateOrFilePrivateScope())
         Scope = getFilePrivateScope(Scope, Decl);
 
     return Scope;