[analyzer] Better modeling of memcpy by the CStringChecker (PR16731).

New rules of invalidation/escape of the source buffer of memcpy: the source buffer contents is invalidated and escape while the source buffer region itself is neither invalidated, nor escape.
In the current modeling of memcpy the information about allocation state of regions, accessible through the source buffer, is not copied to the destination buffer and we can not track the allocation state of those regions anymore. So we invalidate/escape the source buffer indirect regions in anticipation of their being invalidated for real later. This eliminates false-positive leaks reported by the unix.Malloc and alpha.cplusplus.NewDeleteLeaks checkers for the cases like

char *f() {
  void *x = malloc(47);
  char *a;
  memcpy(&a, &x, sizeof a);
  return a;
}

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@194953 91177308-0d34-0410-b5e6-96231b3b80d8