Cherrypick the refcount fixes and comments from mig-review.

I also find the revised "createPart2" values in *Lookup* calls appealing,
but I'm too fearful of breaking things without understanding the implications.
This approach just focuses on fixing the things I broke, without trying
to "fix" things long "broken" in multiple years of releases.
Cherrypick again.  Add test.
Fix for Bug 3602706.
[Bug 3433012]: Added dummy version of TclpLoadMemory to use in the event that a
platform thinks it can load from memory but cannot actually do so due to it
being disabled at configuration time.
Merge core-8-5-branch.
If the string representation is invalidated, the Unicode intRep must be invalidated too.