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

These flags are usable on floating point arithmetic, as well as call,
select, and phi instructions whose resulting type is floating point, or
a vector of, or an array of, a valid type. Whether or not the flags are
valid for a given instruction can be checked with the new
LLVMCanValueUseFastMathFlags function.

These are exposed using a new LLVMFastMathFlags type, which is an alias
for unsigned. An anonymous enum defines the bit values for it.

Tests are added in echo.ll for select/phil/call, and the floating point
types in the new float_ops.ll bindings test.

Select and the floating point arithmetic instructions were not
implemented in llvm-c-test/echo.cpp, so they were added as well.