[ASTMatchers] Add support for floatLiterals

Summary:
Needed to support something like "floatLiteral(equals(1.0))". The
parser for floating point numbers is kept simple, so instead of ".1" you
have to use "0.1".

Reviewed By: aaron.ballman

Differential Revision: https://reviews.llvm.org/D33135

llvm-svn: 305021