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

[Vlad Serebrennikov via cfe-commits]

Endilll 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 

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