[libc++] Make sure that the symbol differ takes into account symbol types

Summary:
Otherwise, it doesn't take into account things like whether the symbol
is defined or undefined, and whether symbols are indirect references
(re-exports) or not.

Reviewers: EricWF

Subscribers: christof, jkorous, dexonsmith, libcxx-commits

Tags: #libc

Differential Revision: https://reviews.llvm.org/D60416

llvm-svn: 358408