When comparing two method overload candidates during overload diagnostics,

When comparing two method overload candidates during overload diagnostics,
skip the object argument conversion if either of the candidates didn't
initialize it.

Fixes PR6421, which is such a very straightforward extension of PR6398 that I
should have worked it into the last test case (and therefore caught it then).
Ah well.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@97135 91177308-0d34-0410-b5e6-96231b3b80d8