[clang][modules] Detect invalidation of SDKSettings.json

[jansvoboda11]

The input file section in module files only stores files loaded into the SourceManager. When rebuilding the module cache, include-trees also include other files, like the SDKSettings.json file. If we don't invalidate the module cache when that file changes, the corresponding include-trees won't agree with the primary TU include-tree on the file contents. This PR makes it so that we do track that file in module files, triggering module cache and therefore include-tree rebuilds, which makes the whole system have a consistent view of the file.

The is implemented by loading the SDKSettings.json file into SourceManager. Note that this doesn't handle all other extra files contained in the include-tree, because I don't think this is a reasonable long-term solution, but the SDKSettings.json file is the one immediately causing issues.

I intentionally suppress reporting of this new file in tests, so that I don't have go updating ~all of them. This file is getting reported by the scanning C API, so that the build system is given the ability to eventually act on this file being out-of-date.

rdar://149868539

[jansvoboda11]

I think the long-term solution is to decouple input files from the SourceManager and have a way to extend the input files written into the PCM with files from the include tree. These are bigger changes that I don't think are reasonable for the upcoming release, so this is just the minimal fix.