[flang] Ignore errors on declarations in interfaces that "have no effect"

Fortran strangely allows declarations to appear in procedure interface
definitions when those declarations do not contribute anything to the
characteristics of the procedure; in particular, one may declare local
variables that are neither dummy variables nor function results.
Such declarations "have no effect" on the semantics of the program,
and that should include semantic error checking for things like
special restrictions on PURE procedures.

Differential Revision: https://reviews.llvm.org/D135209