[analyzer] mark returns of functions where the region passed as parameter was not initialized

In the wild, many cases of null pointer dereference, or uninitialized
value read occur because the value was meant to be initialized by the
inlined function, but did not, most often due to error condition in the
inlined function.
This change highlights the return branch taken by the inlined function,
in order to help user understand the error report and see why the value
was uninitialized.

rdar://36287652

Differential Revision: https://reviews.llvm.org/D41848

llvm-svn: 325976