[tagolog]

To tell whether an open file is on a local disk, is_local resolved
its handle to a path with GetFinalPathNameByHandleW, found the volume
with GetVolumePathNameW and classified it with GetDriveTypeW.
This chain is pretty expensive. GetVolumePathNameW alone
takes 0.2-0.4 ms. mapped_file_region does this check on every unmap,
slowing down the on-disk CAS.

Ask the volume through the handle first. NtQueryVolumeInformationFile
returns the device type in one call. DRIVE_FIXED means the drive is
a fixed local disk. Any other answer, or a failed query, falls back to
the path-based check as before.

A cache hit of a single-file compile is 2.1% faster through the
dependency scanning daemon and 1.4% faster in-process. Measured with
clang compile caching on the on-disk CAS in the swiftlang/llvm-project
fork.

[github-actions]

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