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

[Nico Weber via cfe-commits]

nico wrote:

> 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?

Finally coming back to this. The problem is that I run tests without vcvarsall.bat run, i.e. I don't have MSVC's tools on PATH. `env -i bin/clang/exe -Ofast -O2 -### -Werrror clang/test/Driver/Ofast.c` then prints "clang: error: unable to find a Visual Studio installation; try running Clang from a developer prompt [-Werror,-Wmsvc-not-found]". It looks like this is the only test that

a) schedules to run the linker and
b) uses -Werror

Just adding a `-c` on line 6 (so that the driver doesn't try to discover a linker binary) makes the test pass on that bot. I'm guessing adding that is fine? Probably also for the other run lines in the test?

(Alternatively, `-fuse-ld=lld` would likely do the trick too, but from what I can tell the test isn't actually trying to test linker-related things.)

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