Eliminate more symbols multiply defined between libsupc++ and libc++.

The remaining multiple definitions were flushed out by attempting to
link libsupc++ and libc++ into the same executable with --whole-archive,
e.g.

clang++ -I../llvm/projects/libcxx/include -nodefaultlibs -Wl,--whole-archive lib/libc++.a /usr/lib/gcc/x86_64-linux-gnu/4.6/libsupc++.a -Wl,--no-whole-archive -lgcc -lgcc_s -lc -lpthread -lrt

(The same technique was used to flush out multiple definitions in
libstdc++.)

Differential Revision: http://llvm-reviews.chandlerc.com/D1824
Make it possible to link against libstdc++ as well as libsupc++ with CMake.

Linking against libstdc++, rather than libsupc++, is probably better
for people who need to link against clients of libstdc++.  Because
libsupc++ is provided only as a static library, its globals are not
shared between the static library and the copy linked into libstdc++.
This has been found to cause at least one test failure.

This also removes a number of symbols which were multiply defined
between libstdc++ and libc++, only when linking with libstdc++.

Differential Revision: http://llvm-reviews.chandlerc.com/D1825
Implement std::exception_ptr under libsupc++.

libsupc++ does not implement the dependent EH ABI and the
functionality it uses to implement std::exception_ptr (which it
declares as an alias of std::__exception_ptr::exception_ptr) is not
directly exported to clients. So we have little choice but to hijack
std::__exception_ptr::exception_ptr's (which fortunately has the
same layout as our std::exception_ptr) copy constructor, assignment
operator and destructor (which are part of its stable ABI), and its
rethrow_exception(std::__exception_ptr::exception_ptr) function.

Also, remove some out of date comments.

Differential Revision: http://llvm-reviews.chandlerc.com/D1826
Silence the unused function warning in exception.cpp.

Rather than try to protect the function behind a precise,
ever-changing #if expression, just inline it into every caller.