[libc++] Instantiate additional <iostream> members in the dylib

This commit adds new explicit instantiations for some classes in <iostream>
in the library. This is done after noticing that many programs that use
streams end up containing weak definitions of these classes, which has a
negative impact on both code size and load times (due to the need to
resolve weak symbols at load time). Note that we are just adding the
additional explicit instantiations for the `char` specializations, since
the `wchar_t` specializations are not used as often, and as a result there
wouldn't be a clear benefit.

This change is not an ABI break, since we are just adding additional
symbols.

Differential Revision: https://reviews.llvm.org/D90677