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

[James Y Knight via cfe-commits]

================
@@ -470,6 +470,9 @@ New Compiler Flags
 Deprecated Compiler Flags
 -------------------------
 
+- ``-Ofast`` is deprecated in favor of ``-O3``, possibly combined with ``-ffast-math``.
----------------
jyknight wrote:

Suggestion:
```
- The ``-Ofast`` command-line option has been deprecated. This option both
  enables the ``-O3`` optimization-level, as well as enabling non-standard
  ``-ffast-math`` behaviors. As such, it is somewhat misleading as an
  "optimization level". Users are advised to switch to ``-O3 -ffast-math`` if
  the use of non-standard math behavior is intended, and ``-O3`` otherwise.
```

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