[llvm] [C API] Add getters and setters for fast-math flags on relevant instructions (PR #75123)

[Benji Smith via llvm-commits]

Benjins wrote:

Pushed a new commit that addresses review feedback:
 - Periods in documentation and formatting
 - Changing `LLVMFastMathAll` to be a bitwise or of all the other flags, rather than `-1`
 - Move `LLVMSetFastMathFlags` to after switch statement in `echo.cpp` for non-phi instructions
 - No target triple in `float_ops.ll`

https://github.com/llvm/llvm-project/pull/75123