[clang] [llvm] [IR] Allow fast math flags on fptrunc and fpext (PR #115894)

[Joshua Cranmer via cfe-commits]

https://github.com/jcranmer-intel approved this pull request.

I think there's some followup work needed to get this working in SelectionDAG as well, but that can live in a separate patch (especially as we need to do a followup pass in DAGCombine to check for flags on the fpext/fptrunc nodes).

https://github.com/llvm/llvm-project/pull/115894