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

[Fangrui Song via cfe-commits]

MaskRay 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'
> ```
> 
> 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: c3a50879dfca9ab06ebfe52f48019bb6ac40bb4d .

The same "diagnostics are suppressed" issue happened recently on two bots that @dyung maintains, but I do not know whether theirs are Windows bots.

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