gh-131296: Fix Windows build warnings

[vstinner]

Fix the following warnings:

• Modules_io\fileio.c(1296,13): unused variable 'self'
• Modules_io\winconsoleio.c(334,9): unused variable 'fd_is_own'
• Modules\faulthandler.c(409,11): unused variable 'flags'
• Modules\posixmodule.c(5699,9): unused variable 'pathError'
• PC\winreg.c(2077,5): suggest braces around initialization of subobject
• PC\winreg.c(34,13): unused variable 'errNotAHandle'
• Python\fileutils.c(132,12): result of comparison of constant 1114111 with expression of type 'wchar_t' (aka 'unsigned short') is always false
• Python\fileutils.c(58,21): unused variable 'INCOMPLETE_CHARACTER'
• Python\sysmodule.c(2534,21): unused variable 'perf_map_state'

[vstinner]

Ah, these compiler warnings are from clang if I understood correctly: https://github.com/python/cpython/actions/runs/13967620949/job/39101561262?pr=131495

[zooba]

these compiler warnings are from clang

Yeah, they will be. MSVC warnings have an error code, while clang is going to show the -W<name> option.

@chris-eibl has been working on clang-cl warnings. They aren't blocking, but probably do show things that are worth fixing. (I'm guessing we've just globally suppressed them on MSVC, as all of those do exist in MSVC.)

[chris-eibl]

Yeah, see #131296
I think you've made some of my PRs I've created some days ago obsolete.
Maybe you've even fixed some more :)

In total, there have been still 40ish warnings before my PRs were merged.

How to proceed?

I'll look through yours and mine and will close the obsolete ones?

[chris-eibl]

@vstinner: I think I've closed all the duplicates and rebased #131304. There is one more open and I plan to do some more - there are still some warnings, but feel free to take over :)

Can you update the issue title so it gets linked with #131296?