<rdar://problem/12806802> Propagate access specifiers for conversion functions to...

<rdar://problem/12806802> Propagate access specifiers for conversion functions to the conversion function set eagerly.

This slightly propagates an existing hack that delays when we provide
access specifiers for the visible conversion functions of a class by
copying the available access specifier early. The only client this
affects is LLDB, which tends to discover and add conversion functions
after the class is technically "complete". As such, the only
observable difference is in LLDB, so the testing will go there.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@179029 91177308-0d34-0410-b5e6-96231b3b80d8