[clang] [Sema] [Modules] Remove unrelated module partitions from suggestion list (PR #187657)

[Nathan Ridge via cfe-commits]

HighCommander4 wrote:

I have to admit this PR has moved into territory which I also don't understand well.

We started with a fairly simple change to `CodeCompleteModuleImport`. Would it make sense to keep the scope of the PR limited to that, and pursue the changes discussed in [this comment](https://github.com/llvm/llvm-project/pull/187657#issuecomment-4124660898) in a follow-up PR?

Also, @mikomikotaishi, could you say more about the changes to `clang/lib/Lex`? Is that something you found you needed to make the test pass? Or to make the behaviour of clangd on your project work as desired?

https://github.com/llvm/llvm-project/pull/187657