[c++-concepts] Restore missing code from previous trunk merge

[Braden Obrzut]

A little bit of code went missing due to the constant expressions code 
being factored out.  Additionally, concepts are currently dependent on 
changes to CPTK_IS_CONVERTIBLE_TO, so this patch restores that.  I'm 
told that this was removed from trunk and that the concepts branch will 
be adapted to follow suit, but for now it fixes a regression with 
compound requirements.

- Braden Obrzut

2015-02-05  Braden Obrzut  <admin@maniacsvault.net>

     * gcc/cp/constexpr.c (potential_constant_expression_1): Readded missing
     cases from previous merge from trunk.
     * gcc/cp/cxx-pretty-print.c (pp_cxx_trait_expression): Restored
     CPTK_IS_CONVERTIBLE_TO for the time being since the constraint code
     relies on it.
     * gcc/cp/semantics.c (finish_trait_expr): Likewise.
     * gcc/testsuite/g++.dg/concepts/req9.C: New test.
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