[cxx-interop] Do not get private discriminator for private Clang types (#80485)

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
(cherry picked from commit dd2f465)

Author: j-hui

lib/IRGen/IRGenDebugInfo.cpp

@@ -2577,8 +2577,13 @@ class IRGenDebugInfoImpl : public IRGenDebugInfo {
 
     // Scope outermost fileprivate decls in an inline private discriminator
     // namespace.
+    //
+    // We need to don't do this for decls imported from Clang modules because
+    // the scopes of C/C++ symbols are not restricted to a particular file unit.
     if (auto *Decl = DbgTy.getDecl())
-      if (Decl->isOutermostPrivateOrFilePrivateScope())
+      if (Decl->isOutermostPrivateOrFilePrivateScope() &&
+          !isa<ClangModuleUnit>(
+              Decl->getDeclContext()->getModuleScopeContext()))
         Scope = getFilePrivateScope(Scope, Decl);
 
     return Scope;

test/Interop/Cxx/class/access/private-fileid-irgen.swift

@@ -1,6 +1,7 @@
 //--- blessed.swift
 // RUN: split-file %s %t
 // RUN: %target-swift-frontend -emit-ir -module-name main %t/blessed.swift -I %S/Inputs -cxx-interoperability-mode=default -Onone | %FileCheck %s
+// RUN: %target-swift-frontend -emit-ir -module-name main %t/blessed.swift -I %S/Inputs -cxx-interoperability-mode=default -Onone -g | %FileCheck %s
 
 import NonPublic