[flang] Initial support for FastMathAttr setup in FirOpBuilder.

Provide FirOpBuilder::setFastMathFlags() to configure FastMathFlags
for the builder. Set FastMathAttr for operations based on FirOpBuilder
configuration via mlir::OpBuilder::Listener.

This is a little bit hacky solution, because we lose the ability
to hook other listeners to FirOpBuilder. There are also potential
issues with OpBuilder::clone() - the hook will be invoked for cloned
operations and will effectively overwrite FastMathAttr with the ones
configured in FirOpBuilder, which should not be happening.
We should teach mlir::OpBuilder about FastMathAttr setup in future.

Reviewed By: jeanPerier, kiranchandramohan

Differential Revision: https://reviews.llvm.org/D137390