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

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

This reverts commit af86957cbbffd3dfff3c6750ebddf118aebd0069.

Close https://github.com/llvm/llvm-project/issues/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 https://github.com/llvm/llvm-project/issues/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.