[flang] Semantic checks for constraints on types

I implemented and added tests for constraints C703, C704, C705, C706,
and C796.  In some cases, the code and/or test already existed, and all
I did was add a notation indicating the associated constraint.

Original-commit: flang-compiler/f18@49a64c4c2374e930f6890b270289a6f49ba63edd
Reviewed-on: https://github.com/flang-compiler/f18/pull/978