[analyzer] CStringChecker: pr34460: Avoid a crash when a cast is not modeled.
The checker used to crash when a mempcpy's length argument is symbolic. In this
case the cast from 'void *' to 'char *' failed because the respective
ElementRegion that represents cast is hard to add on top of the existing
ElementRegion that represents the offset to the last copied byte, while
preseving a sane memory region structure.
Additionally, a few test cases are added (to casts.c) which demonstrate problems
caused by existing sloppy work we do with multi-layer ElementRegions. If said
cast would be modeled properly in the future, these tests would need to be
taken into account.
Differential Revision: https://reviews.llvm.org/D38797
git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@315742
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