[pseudo] Key guards by RuleID, add guards to literals (and 0).

After this, NUMERIC_CONSTANT and strings should parse only one way.

There are 8 types of literals, and 24 valid (literal, TokenKind) pairs.
This means adding 8 new named guards (or 24, if we want to assert the token).

It seems fairly clear to me at this point that the guard names are unneccesary
indirection: the guards are in fact coupled to the rule signature.

(Also add the zero guard I forgot in the previous patch.)

Differential Revision: https://reviews.llvm.org/D130066