[libc++abi] Disable libc++ extern templates project-wide

libc++abi can't depend on libc++, so disable extern templates in libc++
headers project-wide. This was previously done in cxa_demangle.cpp, but
I consider it more appropriate to do at the cmake level (since none of
libc++abi's source files can use libc++ extern templates).

I also think the _LIBCPP_NO_EXCEPTIONS in cxa_demangle.cpp is
suspicious, but I'm doing one thing at a time.

Differential Revision: https://reviews.llvm.org/D32329

llvm-svn: 302739