More accurately model realloc() when the size argument is 0. realloc() with a size...

More accurately model realloc() when the size argument is 0. realloc() with a size of 0 is equivalent to free(). The memory region should be marked as free and not used again.

Unit tests f2_realloc_0(), f6_realloc(), and f7_realloc() contributed by Marshall Clow <mclow.lists@gmail.com>. Thanks!

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@130303 91177308-0d34-0410-b5e6-96231b3b80d8