[flang] Fix bug making symbol for external procedure

When we encounter a ProcedureDesignator, if it is an external procedure
(i.e. there is no visible declaration) it must be implicitly created in
the global scope, not the current scope.

If there is already a symbol there (i.e. a module) it is an error, even
if that symbol is not visible due to an IMPORT statement.

Original-commit: flang-compiler/f18@835e234ded3f4f9b47565c46fcbf42e25e966550
Reviewed-on: https://github.com/flang-compiler/f18/pull/249
Tree-same-pre-rewrite: false