[flang] Improvements to name resolution in data statements

Handle variables in data statements differently from implied dos:
If a variable is first mentioned in a data statement, it is implicitly
declared in the current scope (and subsequent type declarations only
confirm the type).
In the implied do case, the only variables that can be implicitly
declared are the iteration variables. They must be created in the new
ImpliedDos scope. There must be one such scope for each data-stmt-object
because the variable name can be reused.

Resolve references to named constants, e.g. in complex-literal-constant
and data-stmt-value. This requires changing the handling of
named-constant-def as that is a definition, not a reference.

Original-commit: flang-compiler/f18@9952b56af090b307f03202c128f242866d76d1a2
Reviewed-on: https://github.com/flang-compiler/f18/pull/277
Tree-same-pre-rewrite: false