[CIR] Implement __builtin_flt_rounds and __builtin_set_flt_rounds (#190706)
This adds CIR handling for the __builtin_flt_rounds and
__builtin_set_flt_rounds builtin functions. Because the LLVM dialect
does not have dedicated operations for these, I have chosen not to
implement them as operations in CIR either. Instead, we just call the
LLVM intrinsic.