[flang] Clean up bogus semantic error on procedure pointer assignment

When a procedure pointer with no interface is associated with
an EXTERNAL name with no interface information, but it is later
inferred that the procedure pointer must be a subroutine because it
appears in a CALL statement, don't complain that the EXTERNAL name
is not also known to be a subroutine.

Subroutine vs. function errors are still caught in procedure pointer
assignment compatibility checking; this fix simply ensures that those
more nuanced tests are not overridded by the attribute set equality test.

Also, leave in some code for dumping the differing attributes in legitimate
error cases that was added in the coures of debugging the specific problem.

Differential Revision: https://reviews.llvm.org/D130385