[Support] Add sys::fs::getRealPathFromHandle (#239019)
The real path of an open file is available only from
`openFileForRead` at open time. A caller that holds an open file
and needs its real path later has no API for it.
Expose that logic as new `sys::fs::getRealPathFromHandle`.
The real path of an open file is asked via a file handle.
API returns `errc::not_supported` when the platform cannot answer
for a handle.
The function comes from `swiftlang/llvm-project`:
https://github.com/swiftlang/llvm-project/pull/5425
---------
Co-authored-by: Steven Wu <stevenwu@apple.com>
