[clang] [Driver] Introduce ffp-model=aggressive (PR #100453)

[Aaron Ballman via cfe-commits]

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@@ -102,6 +102,16 @@ Deprecated Compiler Flags
 Modified Compiler Flags
 -----------------------
 
+- The ``-ffp-model`` option has been updated to enable a more limited set of
+  optimizations when the ``fast`` argument is used and to accept a new argument,
+  ``aggressive``. The behavior of ``-ffp-model=aggressive`` is equivalent
+  to the previous behavior of ``-ffp-model=fast``. The updated
+  ``-ffp-model=fast`` behavior no longer assumes finite math only, uses
+  the ``promoted`` algorithm for complex division when possible rather than the
+  less basic (limited range) algorithm, and sets
+  ``-ffp-contract=fast-honor-pragmas`` rather than ``-ffp-contract=fast``.
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AaronBallman wrote:

I'm not super comfortable with this approach because it's a silent behavior change between compiler releases; would it be reasonable to deprecate "fast" entirely (leaving its behavior alone) and introduce two new options: `aggressive` and `user-hostile` (or something perhaps less loaded, lol)?

https://github.com/llvm/llvm-project/pull/100453