[mlir][LLVMIR] Update LLVMIR fastmath to use EnumAttr tblgen classes

This diff updates the `fastmath` attribute in the LLVMIR dialect to use `tblgen`
classes that were developed after the initial LLVMIR `fastmath` implementation.
Using the `EnumAttr` `tblgen` classes brings the LLVMIR `fastmath` attribute in
line with other dialects, and eliminates some of the custom printing and parsing
code in the LLVMIR dialect.

Subsequent commits will further reduce the custom processing code for the LLVMIR
`fastmath` attribute by unifying printing/parsing functionality between the
LLVMIR and `arith` `fastmath` attributes. (The actual attributes will remain
separate, but the printing and parsing will be made generic, and will be usable
by other dialects/attributes.)

Reviewed By: ftynse

Differential Revision: https://reviews.llvm.org/D135289