[mlir] Add ConstantLike trait to LLVM::ConstantOp

This make LLVM dialect constants to work with `m_constant` matches. Implement
the folding hook for this operation as required by the trait. This in turn
allows LLVM::ConstantOp to properly participate in constant-folding.

Depends On D116757

Reviewed By: wsmoses

Differential Revision: https://reviews.llvm.org/D116758