Pull in r242623 from upstream libc++ trunk (by Eric Fiselier):

Pull in r242623 from upstream libc++ trunk (by Eric Fiselier):

  Enable and fix warnings during the build.

  Although CMake adds warning flags, they are ignored in the libc++ headers
  because the headers '#pragma system header' themselves.

  This patch disables the system header pragma when building libc++ and fixes
  the warnings that arose.

  The warnings fixed were:
  1. <memory> - anonymous structs are a GNU extension
  2. <functional> - anonymous structs are a GNU extension.
  3. <__hash_table> - Embedded preprocessor directives have undefined behavior.
  4. <string> - Definition is missing noexcept from declaration.
  5. <__std_stream> - Unused variable.

This should fix building world (in particular libatf-c++) with -std=c++11.

Reported by:	Oliver Hartmann <ohartman@zedat.fu-berlin.de>