feature: Per-module -fmodules-decluse (-fmodules-decluse=<module_name>)

I’m setting up Clang module maps for Qt and thought I’d use -fmodules-decluse to test my work. Unfortunately, enabling -fmodules-decluse results in quite a few errors of this kind:

```
While building module '_DarwinFoundation2' imported from /Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX.sdk/usr/include/stdio.h:61:
In file included from :1:
/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX.sdk/usr/include/nl_types.h:44:10: error: module _DarwinFoundation2 does not directly depend on a module exporting 'sys/cdefs.h', which is part of indirectly-used module cdefs
44 | #include
| ^
```

Which doesn't seem possible to work around.

As suggesteded in https://discourse.llvm.org/t/fmodules-decluse-resulting-in-errors-from-xcode-sdk/88267 a `-fmodules-decluse=` might be a useful addition to the modules machinery.

Thanks!

Start in the Clang modules machinery and reproduce the reported errors with Qt module maps, the Xcode SDK, and -fmodules-decluse. Compare the proposed -fmodules-decluse= behavior with the existing flag, then verify that per-module declaration-use checking handles indirectly used modules without the reported diagnostics.

Written by the indexing model from the issue text.