[TableGen] Update LLVM TableGen documentation with some of the common errors and its possible meaning

[pchaudhuri16]

I have observed that some of the errors encountered while working with LLVM TableGen are difficult to understand, especially if one is new to TableGen. Hence in my personal opinion, it may be helpful to document some of these errors with possible meaning and solution on https://llvm.org/docs/TableGen/BackGuide.html#debugging-tools.

I have documented few of the errors in this excel sheet - https://docs.google.com/spreadsheets/d/125yhPn-drS4OWUGVwY26fQzVVyC42VN8UhG_DK8ws78/edit?gid=694808853#gid=694808853