Fix issue 10165 (rework of PR #1302)

http://issues.dlang.org/show_bug.cgi?id=10165
Issue 10165: No syntax to create thread-local shared variables

Adds `UnqualRef` templated alias/struct that is similar to Rebindable but
strips away all qualifiers completely (including shared)

An alternative to https://github.com/D-Programming-Language/phobos/pull/1302
`U` being usable in the *else-clause* like this is kind of spooky. It works out here because the static-if happens to use negation, but this doesn't make any sense in general. I think the condition should be inverted.