The GNU-style aligned attribute has an optional expression, but the generated pretty printing logic was unaware of this. Fixed the pretty printing logic, and added a test to ensure it no longer asserts.

Added a FIXME to the code about eliding the parenthesis when pretty printing such a construct.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@214513 91177308-0d34-0410-b5e6-96231b3b80d8