[Syntax] Tablegen literal expressions.

Non-mechanical changes:
 - Added FIXME to StringLiteral to cover multi-token string literals.
 - LiteralExpression::getLiteralToken() is gone. (It was never called)
   This is because we don't codegen methods in Alternatives
   It's conceptually suspect if we consider multi-token string literals, though.

Differential Revision: https://reviews.llvm.org/D91277