[analyzer] Show "Returning from ..." note at caller's depth, not callee's.

Before:
  1. Calling 'foo'
    2. Doing something interesting
    3. Returning from 'foo'
  4. Some kind of error here

After:
  1. Calling 'foo'
    2. Doing something interesting
  3. Returning from 'foo'
  4. Some kind of error here

The location of the note is already in the caller, not the callee, so this
just brings the "depth" attribute in line with that.

This only affects plist diagnostic consumers (i.e. Xcode). It's necessary
for Xcode to associate the control flow arrows with the right stack frame.

<rdar://problem/13634363>

llvm-svn: 179351