Clean up empty struct/union recognition.

Clean up empty struct/union recognition.
Make use of getTypeSizeInChars to detect structs/unions of zero size. It allows
more accurate detection of types of zero size. It however has a side effect -
sequence of used types may change, that is why the test 'override-layout' was
modified.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@184088 91177308-0d34-0410-b5e6-96231b3b80d8