P1152R4: Fix deprecation warnings in libc++ testsuite and in uses of...

P1152R4: Fix deprecation warnings in libc++ testsuite and in uses of is_invocable that would internally conjure up a deprecated function type.

Summary: The implementation of P1152R4 in Clang has resulted in some deprecation warnings appearing in the libc++ and libc++abi test suite. Fix or suppress these warnings.

Reviewers: mclow.lists, EricWF

Subscribers: christof, ldionne, libcxx-commits

Tags: #libc

Differential Revision: https://reviews.llvm.org/D68879

llvm-svn: 375307