[clang-cl] Increase /fp flag fidelity

They were mapped as follows:
- /fp:except to --ftrapping-math
- /fp:except- to --fno-trapping-math
- /fp:strict to --fno-fast-math
- /fp:precise to --fno-fast-math

Let's map them as follows:
- /fp:except to --ffp-exception-behavior=strict
- /fp:except- to --ffp-exception-behavior=ignore
- /fp:strict to --ffp-model=strict
- /fp:precise to --ffp-model=ignore

I believe the changes to /fp:except are technically a no-op but it makes
the mapping a lot clearer. The changes for /fp:strict and /fp:precise are not
no-ops, they now match MSVC's behavior.

While we are here, also add support for /fp:contract by mapping it to
-ffp-contract=on.