Revert "[C++20] [Modules] Don't load declaration eagerly for named modules"

This reverts commit af86957.

Close #61733.

Previously I banned the eagerly loading for declarations from named
modules to speedup the process of reading modules. But I didn't think
about special decls like PragmaCommentDecl and PragmaDetectMismatchDecl.
So here is the issue #61733.

Note that the current behavior is still incorrect. Given:

```
// mod.cppm
module;

export module mod;
```

and

```
// user.cpp
import mod;
```

Now the IR of `user.cpp` will contain the metadata '!0 =
!{!"msvcprt.lib"}' incorrectly. The root cause of the problem is that
`EagerlyDeserializedDecls` is designed for headers and it didn't take
care for named modules. We need to redesign a new mechanism for named
modules.

Author: ChuanqiXu9

clang/lib/Serialization/ASTWriterDecl.cpp

@@ -2468,13 +2468,7 @@ void ASTWriter::WriteDeclAbbrevs() {
 /// relatively painless since they would presumably only do it for top-level
 /// decls.
 static bool isRequiredDecl(const Decl *D, ASTContext &Context,
-                           Module *WritingModule) {
-  // Named modules have different semantics than header modules. Every named
-  // module units owns a translation unit. So the importer of named modules
-  // doesn't need to deserilize everything ahead of time.
-  if (WritingModule && WritingModule->isModulePurview())
-    return false;
-
+                           bool WritingModule) {
   // An ObjCMethodDecl is never considered as "required" because its
   // implementation container always is.