[flang] Clause 12 semantics -- Check all constraints not otherwise checked (flang-compiler/f18#427)

* Clause 12 semantics

Check all constraints not otherwise checked during parsing or label scope
validation, except for C1201, C1231, and C1233-5.  Obvious program
requirements are also checked, except for 12.6.2.2 constant format string
validation.

Original-commit: flang-compiler/f18@e4ec3436184935c0c09f0b61c01f3cb86d84d0b3
Reviewed-on: https://github.com/flang-compiler/f18/pull/427