Fix a vftable mangling bug

We need to walk the class hierarchy twice: once in depth-first base
specifier order for mangling and again in depth-first layout order for
vftable layout.

Vftable layout seems to depend on the full path from the most derived
class to the base containing the vfptr.

Fixes PR21031.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@218285 91177308-0d34-0410-b5e6-96231b3b80d8