[Fixed Point] [AST] Add an AST serialization code for fixed-point literals.

Summary:
This patch adds the EXPR_FIXEDPOINT_LITERAL AST
code to serialize FixedPointLiterals. They were previously
being serialized with the code for integer literals, which
doesn't work properly.

Reviewers: leonardchan, rjmccall

Reviewed By: leonardchan, rjmccall

Subscribers: vabridgers, JesperAntonsson, bjope, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D57226