[analyzer] Calculate extent size for memory regions allocated by new expression.

ArrayBoundChecker did not detect out of bounds memory access errors in case an
array was allocated by the new expression. This patch resolves this issue.

Patch by Daniel Krupp!

Differential Revision: https://reviews.llvm.org/D24307



git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@281934 91177308-0d34-0410-b5e6-96231b3b80d8