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

llvm-svn: 354515