[flang] Check semantic contraints for the nullify statement. The full

[flang] Check semantic contraints for the nullify statement. The full
legality check is up to the user; however, some checks could
be done at compile time and others at runtime.  We choose to
skip any checking at compile time because it would not be
complete.

Note the instantiation of ExtractDataRef in expressions.cc.
This instantiation satifies the reference in check-nullify.cc
because expression.h just has a declaration of ExtractDataRef
instead of the definition.

Original-commit: flang-compiler/f18@b7199219fbf9e833be6e745d498e99936c2e68b2
Reviewed-on: https://github.com/flang-compiler/f18/pull/388
Tree-same-pre-rewrite: false