[MLIR,OpenMP] Lowering of parallel operation: proc_bind clause 2/n

This patch adds the translation of the proc_bind clause in a
parallel operation.

The values that can be specified for the proc_bind clause are
specified in the OMP.td tablegen file in the llvm/Frontend/OpenMP
directory. From this single source of truth enumeration for
proc_bind is generated in llvm and mlir (used in specification of
the parallel Operation in the OpenMP dialect). A function to return
the enum value from the string representation is also generated.
A new header file (DirectiveEmitter.h) containing definitions of
classes directive, clause, clauseval etc is created so that it can
be used in mlir as well.

Reviewers: clementval, jdoerfert, DavidTruby

Differential Revision: https://reviews.llvm.org/D84347