[mlir][Tablegen-LSP] Add support for a tracking definitions and references

This essentially builds an index for the parsed records and record values (fields).
This covers quite a few cases, but is limited by the currently lackluster location
tracking in tablegen. A followup will work on plumbing more locations through
tablegen, which should greatly improve what we can do here.

Differential Revision: https://reviews.llvm.org/D125443