Refactor places which perform contextual implicit conversions to go through a
common function. The C++1y contextual implicit conversion rules themselves are
not yet implemented, however.

This also fixes a subtle bug where template instantiation context notes were
dropped for diagnostics coming from conversions for integral constant
expressions -- we were implicitly slicing a SemaDiagnosticBuilder into a
DiagnosticBuilder when producing these diagnostics, and losing their context
notes in the process.