[flang] Support arith::FastMathFlagsAttr for fir::CallOp.

The main purpose of this patch is to propagate fastmath attribute
to SimplifyIntrinsicsPass, so that the inline code can inherit
the call operation's attributes. Even though I added translation
of fastmath from fir::CallOp to LLVM::CallOp, there are no fastmath
attributes in LLVM IR. It looks like the translation drops it.
This will need additional commits.

Reviewed By: jeanPerier

Differential Revision: https://reviews.llvm.org/D137602