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

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 <[email protected]>

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