[C++20] [Modules] [Serialization] Don't record '#pragma once' informa…

…tion in named modules

Close #77995

The cause of the issue is that straight forward that we recorded the
'#pragma once' information in named modules, which is an overlook.

I tried to not record the header's information completely within named
modules. But the tests look not happy with some diagnosing problems,
which needs to be looked in details.

clang/lib/Serialization/ASTWriter.cpp

@@ -1897,7 +1897,8 @@ namespace {
 
       unsigned char Flags = (Data.AlreadyIncluded << 6)
                           | (Data.HFI.isImport << 5)
-                          | (Data.HFI.isPragmaOnce << 4)
+                          | (Writer.isWritingStdCXXNamedModules() ? 0 :
+                             Data.HFI.isPragmaOnce << 4)
                           | (Data.HFI.DirInfo << 1)
                           | Data.HFI.IndexHeaderMapHeader;
       LE.write<uint8_t>(Flags);

clang/test/Modules/pr77995.cppm

@@ -0,0 +1,25 @@
+// RUN: rm -rf %t
+// RUN: mkdir -p %t
+// RUN: split-file %s %t
+//
+// RUN: %clang_cc1 -std=c++20 %t/foo.cppm -emit-module-interface -o %t/foo.pcm
+// RUN: %clang_cc1 -std=c++20 %t/use.cpp -fmodule-file=foo=%t/foo.pcm -verify -fsyntax-only
+
+//--- a.hpp
+#pragma once
+#define A 43
+
+//--- foo.cppm
+module;
+#include "a.hpp"
+export module foo;
+export constexpr auto B = A;
+
+//--- use.cpp
+// expected-no-diagnostics
+import foo;
+#include "a.hpp"
+
+static_assert(A == 43);
+static_assert(B == 43);
+