[mlir] Add hover support to mlir-lsp-server

This provides information when the user hovers over a part of the source .mlir file. This revision adds the following hover behavior:
* Operation:
  - Shows the generic form.
* Operation Result:
  - Shows the parent operation name, result number(s), and type(s).
* Block:
  - Shows the parent operation name, block number, predecessors, and successors.
* Block Argument:
  - Shows the parent operation name, parent block, argument number, and type.

Differential Revision: https://reviews.llvm.org/D101113