The compiler already has special support for fixing up verbatim paths with disks
on Windows to something that can be correctly passed down to gcc, and this
commit adds support for verbatim UNC paths as well.
Closes#25505
Turns out that a verbatim path was leaking through to gcc via the PATH
environment variable (pointing to the bundled gcc provided by the main
distribution) which was wreaking havoc when gcc itself was run. The fix here is
to just stop passing verbatim paths down by adding more liberal uses of
`fix_windows_verbatim_for_gcc`.
Closes#25072
