Rewrite the tblgen parser in a recursive descent style, eliminating the bison parser.

Rewrite the tblgen parser in a recursive descent style, eliminating the bison parser.

This makes the parser much easier to understand, eliminates a ton of global variables,
and gives tblgen nice caret diagnostics.  It is also faster, but tblgen probably doesn't
care about performance.

There are a couple of FIXMEs which I will take care of next.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@44274 91177308-0d34-0410-b5e6-96231b3b80d8