[C++20] [Modules] Skip ODR checks if either declaration comes from GMF

This patch tries to workaround the case that:
- in a module unit that imports another module unit
- both the module units including overlapped headers
- the compiler emits false positive ODR violation diagnostics for the
  overlapped headers if ODR check is enabled
- the current module units enables PCH

For the third point, we disabled ODR check if the declarations comes
from GMF. However, due to the forth point, the check whether the
declaration comes from GMF failed. Then we still going to check it and
then the users get false positive checks.

What's worse is that, this always happens in clangd, where will generate
the PCH automatically before parsing the input files.

The root cause of the problem we mixed the modules in the semantical
level and the module in the serialization level.

The problem is pretty fundamental and we need time to fix that. But 19.x
is going to be branched and I hope to give clangd better user
experience. So I decided to land this workaround even if it is pretyy
niche and may only work for the case of clangd's pattern.

clang/lib/Serialization/ASTReader.cpp

@@ -9875,6 +9875,7 @@ void ASTReader::finishPendingActions() {
             // We only perform ODR checks for decls not in the explicit
             // global module fragment.
             !shouldSkipCheckingODR(FD) &&
+            !shouldSkipCheckingODR(NonConstDefn) &&
             FD->getODRHash() != NonConstDefn->getODRHash()) {
           if (!isa<CXXMethodDecl>(FD)) {
             PendingFunctionOdrMergeFailures[FD].push_back(NonConstDefn);

clang/lib/Serialization/ASTReaderDecl.cpp

@@ -821,7 +821,7 @@ void ASTDeclReader::VisitEnumDecl(EnumDecl *ED) {
       Reader.mergeDefinitionVisibility(OldDef, ED);
       // We don't want to check the ODR hash value for declarations from global
       // module fragment.
-      if (!shouldSkipCheckingODR(ED) &&
+      if (!shouldSkipCheckingODR(ED) && !shouldSkipCheckingODR(OldDef) &&
           OldDef->getODRHash() != ED->getODRHash())
         Reader.PendingEnumOdrMergeFailures[OldDef].push_back(ED);
     } else {
@@ -2134,7 +2134,7 @@ void ASTDeclReader::MergeDefinitionData(
   }
 
   // We don't want to check ODR for decls in the global module fragment.
-  if (shouldSkipCheckingODR(MergeDD.Definition))
+  if (shouldSkipCheckingODR(MergeDD.Definition) || shouldSkipCheckingODR(D))
     return;
 
   if (D->getODRHash() != MergeDD.ODRHash) {

clang/test/Modules/pch-in-module-units.cppm

@@ -0,0 +1,51 @@
+// Test that we will skip ODR checks for declarations from PCH if they
+// were from GMF.
+//
+// RUN: rm -rf %t
+// RUN: mkdir -p %t
+// RUN: split-file %s %t
+//
+// RUN: %clang_cc1 -std=c++20 -emit-reduced-module-interface %t/A.cppm \
+// RUN:   -o %t/A.pcm -fskip-odr-check-in-gmf
+// RUN: %clang_cc1 -std=c++20 -DDIFF -x c++-header %t/foo.h \
+// RUN:   -emit-pch -o %t/foo.pch -fskip-odr-check-in-gmf
+// RUN: %clang_cc1 -std=c++20 %t/B.cppm -fmodule-file=A=%t/A.pcm -include-pch \
+// RUN:   %t/foo.pch -verify -fsyntax-only -fskip-odr-check-in-gmf
+
+//--- foo.h
+#ifndef FOO_H
+#define FOO_H
+inline int foo() {
+#ifndef DIFF
+  return 43;
+#else
+  return 45;
+#endif
+}
+
+class f {
+public:
+  int mem() {
+#ifndef DIFF
+    return 47;
+#else
+    return 45;
+#endif
+  }
+};
+#endif
+
+//--- A.cppm
+module;
+#include "foo.h"
+export module A;
+export using ::foo;
+export using ::f;
+
+//--- B.cppm
+// expected-no-diagnostics
+module;
+#include "foo.h"
+export module B;
+import A;
+export int b = foo() + f().mem();