[clang] [clang] Add deprecation warning for `-Ofast` driver option (PR #98736)

[via cfe-commits]

dyung wrote:

> > > This seems to break tests on my Windows box: http://45.33.8.238/win/91548/step_6.txt
> > 
> > 
> > This is the failed test:
> > ```
> > c:\src\llvm-project\out\gn\bin\clang.exe -Ofast -O2 -### -Werror C:\src\llvm-project\clang\test\Driver\Ofast.c 2>&1 | c:\src\llvm-project\out\gn\bin\filecheck.exe -check-prefix=CHECK-OFAST-O2    --check-prefix=CHECK-OFAST-O2-ALIASING-MSVC  C:\src\llvm-project\clang\test\Driver\Ofast.c
> > # executed command: 'c:\src\llvm-project\out\gn\bin\clang.exe' -Ofast -O2 '-###' -Werror 'C:\src\llvm-project\clang\test\Driver\Ofast.c'
> > # note: command had no output on stdout or stderr
> > # error: command failed with exit status: 1
> > # executed command: 'c:\src\llvm-project\out\gn\bin\filecheck.exe' -check-prefix=CHECK-OFAST-O2 --check-prefix=CHECK-OFAST-O2-ALIASING-MSVC 'C:\src\llvm-project\clang\test\Driver\Ofast.c'
> > ```
> > 
> > 
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> > Somehow `clang.exe -Ofast -O2 -### -Werror C:\src\llvm-project\clang\test\Driver\Ofast.c 2>&1` is not producing any output for you, despite `-###` being present. I'm not sure why, but all windows buildbots seem to be happy with this patch. Can you investigate further?
> > CC @AaronBallman @MaskRay
> 
> One possibility is that -Wno-msvc-not-found is relevant: [c3a5087](https://github.com/llvm/llvm-project/commit/c3a50879dfca9ab06ebfe52f48019bb6ac40bb4d) . (I think the -Wno-msvc-not-found behavior should be reworked, either removing the driver diagnostic , or delaying the diagnostic after everything else has been reported.)
> 
> The same "diagnostics are suppressed" issue happened recently on two bots that @dyung maintains, but I do not know whether theirs are Windows bots.

For my bots, the bot description should tell you what the bot is running, but I only maintain two builders that run Windows at the moment, but they are the same configuration, only one is a release branch version of the builder, but they run on the same physical worker. Do you have a link to the failures?

https://github.com/llvm/llvm-project/pull/98736