Pull in r354515 from upstream libc++ trunk:

Pull in r354515 from upstream libc++ trunk:

  Fix the build with gcc when `-Wredundant-decls` is passed

  Summary:

  gcc warns that `__throw_runtime_error` is declared both in
  `<__locale>` and `<stdexcept>`, if `-Wredundant-decls` is passed on
  the command line; this is the case with FreeBSD when ${WARNS} == 6.

  Since `<__locale>` gets its first declaration via a transitive
  include of `<stdexcept>`, and the second declaration is after the
  first invocation of `__throw_runtime_error`, delete that second
  declaration.

  Signed-off-by: Enji Cooper <yaneurabeya@gmail.com>
  Reviewers: kristina, MaskRay, EricWF, ldionne, ngie

  Reviewed By: EricWF

  Subscribers: krytarowski, brooks, emaste, dim, christof, jdoerfert, libcxx-commits

  Tags: #libc

  Differential Revision: https://reviews.llvm.org/D58425

Submitted by:	ngie
MFC after:	3 days