[clang] [clang] Make fp contract(on) not overwrite earlier contract(fast) and CLI --ffp-contract=fast(-honor-pragmas) (PR #169859)

[Mikołaj Piróg via cfe-commits]

mikolaj-pirog wrote:

My understanding of interactions of this flag is based on treating `on` as a subset of `fast` -- so when we enable `on`, while under `fast` (via earlier pragma or CLI), we shouldn't overwrite effects of `fast`. Current behavior is surprising to me

https://github.com/llvm/llvm-project/pull/169859