[C++20] [Modules] Don't skip pragma diagnostic mappings

Close #75057

Previously, I thought the diagnostic mappings is not meaningful with
modules incorrectly. And this problem get revealed by another change
recently. So this patch tried to rever the previous "optimization"
partially.

Author: ChuanqiXu9

clang/lib/Serialization/GeneratePCH.cpp

@@ -114,7 +114,6 @@ void CXX20ModulesGenerator::HandleTranslationUnit(ASTContext &Ctx) {
       getPreprocessor().getHeaderSearchInfo().getHeaderSearchOpts();
   HSOpts.ModulesSkipDiagnosticOptions = true;
   HSOpts.ModulesSkipHeaderSearchPaths = true;
-  HSOpts.ModulesSkipPragmaDiagnosticMappings = true;
 
   PCHGenerator::HandleTranslationUnit(Ctx);
 

clang/test/Modules/pr75057.cppm

@@ -0,0 +1,62 @@
+// RUN: rm -rf %t
+// RUN: mkdir -p %t
+// RUN: split-file %s %t
+//
+// Treat the behavior of using headers as baseline.
+// RUN: %clang_cc1 -std=c++20 %t/use-header.cc -isystem %t -fsyntax-only -verify
+//
+// RUN: %clang_cc1 -std=c++20 %t/a.cppm -isystem %t -emit-module-interface -o %t/a.pcm
+// RUN: %clang_cc1 -std=c++20 %t/use-module.cc -isystem %t -fmodule-file=a=%t/a.pcm -fsyntax-only -verify
+
+//--- sys.h
+#ifndef SYS_H
+#define SYS_H
+
+#pragma GCC system_header
+
+template <class C>
+struct [[deprecated]] iterator {};
+
+_Pragma("GCC diagnostic push")
+_Pragma("GCC diagnostic ignored \"-Wdeprecated\"")                         
+_Pragma("GCC diagnostic ignored \"-Wdeprecated-declarations\"")
+
+template <class C>
+struct reverse_iterator 
+: public iterator<C> {};
+
+_Pragma("GCC diagnostic pop")
+
+template <class T>
+class C {
+public:
+    void i() {
+        reverse_iterator<T> i;
+    }
+};
+
+#endif
+
+//--- use-header.cc
+// expected-no-diagnostics
+// However, we see unexpected warnings
+#include <sys.h>
+
+void use() {
+    C<int>().i();
+}
+
+//--- a.cppm
+module;
+#include <sys.h>
+export module a;
+export using ::iterator;
+export using ::C;
+
+//--- use-module.cc
+// expected-no-diagnostics
+import a;
+
+void use() {
+    C<int>().i();
+}