[Clang] Fix strict weak ordering in ItaniumVTableBuilder

In sorting elements can compare with themselves and sometimes assert
further down the line was triggered.

The changes are somewhat NFC, which explains the lack of test coverage.
libc++ has a debug mode that enables extra precondition checking. When
Clang is built with libc++ in that special mode, a few of Clang's tests
would fail with the libc++ assertion because Clang was not honoring the
preconditions for std::stable_sort. However, Clang would not hit the
precondition failure with any release mode STL, so the changes have no
impact on users beyond ones in this very special circumstance.

Differential Revision: https://reviews.llvm.org/D155809