Mark basic_string::assign templates as inline to improve ABI stability.

Visible definitions for basic_string::assign are sometimes emitted in
the dylib depending on the version of LLVM used to compile libc++.
This can cause the check-cxx-abilist target to fail.

This patch attempts marks the basic_string::assign templates as inline
to prevent this. That way the export list is consistent across LLVM
versions.

llvm-svn: 294100