[MS ABI] Use 'continue' instead of 'return false' where intended

This was a bug in r218285 that prevented us from seeing subsequent
virtual bases in the class hierarchy, leading to crashes later.

Also add some comments to this function, now that we better understand
what it's trying to do.

Fixes PR21062 and PR21064.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@235899 91177308-0d34-0410-b5e6-96231b3b80d8