[analyzer][ReturnPtrRangeChecker] Fix a false positive on end() iterator

ReturnPtrRange checker emits a report if a function returns a pointer which
points out of the buffer. However, end() iterator of containers is always such
a pointer, so this always results a false positive report. This false positive
case is now eliminated.

This patch resolves these tickets:
https://bugs.llvm.org/show_bug.cgi?id=20929
https://bugs.llvm.org/show_bug.cgi?id=25226
https://bugs.llvm.org/show_bug.cgi?id=27701

Patch by Tibor Brunner!

Differential Revision: https://reviews.llvm.org/D83678