[TableGen:LSP] Resolve "go-to-def" on a let field to the base definition

This allows for go-to-def on the a `let` field to resolve to the definition
of the base class. This is kind of like how C++ works with go-to-def
from use->def->decl, with the decl in this case being the base definition
of the field.

Differential Revision: https://reviews.llvm.org/D134264