[mlir][llvm] Fastmath flags import from LLVM IR.

This revision adds support to import fastmath flags from LLVMIR. It
implement the import using a listener attached to the builder. The
listener gets notified if an operation is created and then checks if
there are fastmath flags to import from LLVM IR to the MLIR. The
listener based approach allows us to perform the import without changing
the mlirBuilders used to create the imported operations.

An alternative solution, could be to update the builders so that they
return the created operation using FailureOr<Operation*> instead of
LogicalResult. However, this solution implies an LLVM IR instruction
always maps to exatly one MLIR operation. While mostly true, there are
already exceptions to this such as the PHI instruciton. Additionally, an
mlirBuilder based solution also further complicates the builder
implementations, which led to the listener based solution.

Depends on D139405

Reviewed By: ftynse

Differential Revision: https://reviews.llvm.org/D139620