Fix __is_fundamental to accept nullptr_t

    Summary: This patch updates the __is_fundamental builtin type trait to return true for nullptr_t.

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

    Subscribers: cfe-commits

    Tags: #clang

    Differential Revision: https://reviews.llvm.org/D67899

llvm-svn: 372624