[flang] Catch more bad DATA statement objects

The data statement variable checker is missing some cases, like expressions
that are not variables.  Run the checker first to enjoy its very specific
error messages, but when it finds no problems, still apply a general
check that an expression is a "variable" and also not a constant expression
at the top level as a backstop.

Differential Revision: https://reviews.llvm.org/D146580