C++ PATCH to handling of template conversion functions

[Jason Merrill]

This was another change that I needed to make to fix the deduction issue 
mentioned in my previous mail, and brought my attention to an existing 
bug.  When considering a template conversion function we don't want to 
allow derived-to-base conversions, but we should still allow 
qualification conversions.  This patch fixes the latter, and gives a 
more helpful diagnostic for the former.

Tested x86_64-pc-linux-gnu, applying to trunk.
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