Provide a way to disable use of extern templates in libc++. This is intended for...

Provide a way to disable use of extern templates in libc++.  This is intended for the clients of libc++, not the libc++ build.  The dylib should always contain the extern templates.  To disable the client needs to put -D'_LIBCPP_EXTERN_TEMPLATE(...)=' on the command line.

llvm-svn: 167486