[libc++] Apply _LIBCPP_EXCLUDE_FROM_EXPLICIT_INSTANTIATION only in classes...

[libc++] Apply _LIBCPP_EXCLUDE_FROM_EXPLICIT_INSTANTIATION only in classes that we have instantiated externally

To make sure all member functions that require it are marked `_LIBCPP_EXCLUDE_FROM_EXPLICIT_INSTANTIATION` I compared the output of `objdump --syms lib/libc++.1.0.dylib` before and after, ignoring addresses.

Reviewed By: #libc, ldionne

Spies: Mordante, libcxx-commits, ldionne, arichardson, mstorsjo

Differential Revision: https://reviews.llvm.org/D150896