[flang] Fix cycle-catcher in procedure characterization

The "seenProcs" sets passed as arguments to the procedure and dummy
procedure characterization routines need to be passed by value so that
local updates to those sets do not become permanent.  They are
presently passed by reference and that has led to bogus errors about
recursively defined procedures in testing.

(It might be faster to pass the sets by reference and undo those local
updates in these functions, but that's error-prone, and the performance
difference is not expected to be detectable in practice.)

Differential Revision: https://reviews.llvm.org/D122439