Make it clearer that <=> will always be stringified

clang++-6.0 warns that in the next C++ standard <=> will be an operator and therefore an atomic
lexical token. I think that's overly zealous, but it's easily worked around by explicitly quoting
the name, which will be stringified before use in any case.

See diffblue#1974 for more details, or
https://bugs.llvm.org/show_bug.cgi?id=36925 for an upstream bug report.

Author: smowton

src/util/irep_ids.def

@@ -41,7 +41,7 @@ IREP_ID_ONE(member_name)
 IREP_ID_TWO(C_member_name, #member_name)
 IREP_ID_TWO(equal, =)
 IREP_ID_TWO(implies, =>)
-IREP_ID_TWO(iff, <=>)
+IREP_ID_TWO(iff, "<=>")
 IREP_ID_ONE(and)
 IREP_ID_ONE(nand)
 IREP_ID_ONE(or)