This PR builds on top of #113984 and attempts to avoid allocating input
file paths eagerly. Instead, the `InputFileInfo` type used by
`ASTReader` now only holds `StringRef`s that point into the PCM file
buffer, and the full input file paths get resolved on demand.
The dependency scanner makes use of this in a bit of a roundabout way:
`ModuleDeps` now only holds (an owning copy of) the short unresolved
input file paths, which get resolved lazily. This can be a big win, I'm
seeing up to a 5% speedup.
9d4837f47c