[clang] [clang][modules] Stop eagerly reading files with diagnostic pragmas (PR #87442)

[via cfe-commits]

alexfh wrote:

Hi Jan, we started seeing a compilation error in a (quite unusual, frankly speaking) code:
```
pigweed/pw_rpc/public/pw_rpc/internal/channel_list.h:24:10: fatal error: 'vector' file not found
   24 | #include PW_RPC_DYNAMIC_CONTAINER_INCLUDE
      |          ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
pigweed/pw_rpc/public/pw_rpc/internal/config.h:207:42: note: expanded from macro 'PW_RPC_DYNAMIC_CONTAINER_INCLUDE'
  207 | #define PW_RPC_DYNAMIC_CONTAINER_INCLUDE <vector>
      |                                          ^~~~~~~~
<scratch space>:73:1: note: expanded from here
   73 | <vector>
      | ^~~~~~~~
```

The code is coming from here: https://pigweed.googlesource.com/pigweed/pigweed/+/refs/heads/main/pw_rpc/public/pw_rpc/internal/channel_list.h#24

This error only reproduces in our modules build.

https://github.com/llvm/llvm-project/pull/87442