[Patch] Minor additional concept check fixes

[Paolo Carlini]

Hi,

I'm going to apply to v7 the below, which fixes a few remaining minor
issues. The most "important" one is that in merge, set_union,
set_symmetric_difference we were missing a check for output iterator
requirements also together with _ValueType2, besides _ValueType1.

Also, just tested the entire set of concept check fixes in mainline too,
will commit soon.

Paolo.

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