[Analyzer] Iterator Checkers: Replace UnknownVal in comparison result by a conjured value
Sometimes the return value of a comparison operator call is
UnkownVal. Since no assumptions can be made on UnknownVal,
this leeds to keeping impossible execution paths in the
exploded graph resulting in poor performance and false
positives. To overcome this we replace unknown results of
iterator comparisons by conjured symbols.
Differential Revision: https://reviews.llvm.org/D70244