Add some overloads so that floating point literals can be AST matched properly.

I am not entirely sure whether the implemented sematics are ideal. In
particular, should floatLiteral(equals(0.5)) match "0.5f" and should
floatLiteral(equals(0.5f)) match "0.5". With the overloads in this
patch, the answer to both questions is yes, but I am happy to change
that.

llvm-svn: 227956