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

[Fangrui Song via cfe-commits]

MaskRay wrote:

> @MaskRay Are you OK with this change?

Sorry for the delay. I've read through https://discourse.llvm.org/t/making-ffp-model-fast-more-user-friendly/78402,  how we ended up with `-ffp-contract=fast-honor-pragmas` beside `=fast` (GCC compatibility, sigh), some notes that ICC/GCC have behavior differences. A different, safer mode for `-ffp-model` (absent from GCC) makes sense to me. 

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