[MLIR][Math] Add round operation

Introduce RoundOp in the math dialect. The operation rounds the operand to the
nearest integer value in floating-point format. RoundOp lowers to LLVM
intrinsics 'llvm.intr.round' or as a function call to libm (round or roundf).

Reviewed By: ftynse

Differential Revision: https://reviews.llvm.org/D127286