[clang][modules] Avoid calling expensive `SourceManager::translateFile()` (#86216)

The `ASTWriter` algorithm for computing affecting module maps uses
`SourceManager::translateFile()` to get a `FileID` from a `FileEntry`.
This is slow (O(n)) since the function performs a linear walk over
`SLocEntries` until it finds one with a matching `FileEntry`.

This patch removes this use of `SourceManager::translateFile()` by
tracking `FileID` instead of `FileEntry` in couple of places in
`ModuleMap`, giving `ASTWriter` the desired `FileID` directly. There are
no changes required for clients that still want a `FileEntry` from
`ModuleMap`: the existing APIs internally use `SourceManager` to perform
the reverse `FileID` to `FileEntry` conversion in O(1).