SI-6937 core type tags are no longer referentially unique

Type tag factory used to evaluate the provided type creator in the
context of the initial mirror in order to maintain referential equality
of instances of standard tags. Unfortunately this evaluation might fail
if the mirror provided doesn't contain the classes being referred to.
Therefore I think we should avoid evaluating type creators there.

Note that failure of evaluation doesn't mean that there's something
bad going on. When one creates a type tag, the correct mirror /
classloader to interpret that tag in might be unknown (like it happens
here). This is okay, and this is exactly what the 2.10.0-M4 refactoring
has addressed.

Something like `res2.typeTag[A].in(currentMirror)` should be okay.
Merge pull request #2349 from scalamacros/ticket/6937

SI-6937 core type tags are no longer referentially unique