This commit adds a new `TableGenParseFile` entry point for tablegen
that parses an input buffer and invokes a callback function with
a record keeper (notably without an output buffer). This kind of entry
point is very useful for tablegen consuming tools that don't create
output, and want invoke tablegen multiple times. The current way
that we interact with tablegen is via relative includes to
TGParser(not great).
Differential Revision: https://reviews.llvm.org/D119899
David Blaikie