[gomp4] Update error messages for c and c++ reductions

[Cesar Philippidis]

This patches updates the c and c++ FEs to report consistent error
messages for invalid reductions involving array elements, struct
members, and class members. Most of those variables were already
rejected by the generic OpenMP code, but this patch makes the error
messages more precise for OpenACC. It also fixes an ICE involving
invalid struct member reductions in c.

I've committed this patch to gomp-4_0-branch.

Cesar
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