Fix __is_signed builtin

    Summary: This patch fixes the __is_signed builtin type trait to work with floating point types and enums. Now, the builtin will return true if it is passed a floating point type and false for an enum type.

    Reviewers: EricWF, rsmith, erichkeane, craig.topper, efriedma

    Subscribers: cfe-commits

    Tags: #clang

    Differential Revision: https://reviews.llvm.org/D67897

llvm-svn: 372621