Consumed analysis: add return_typestate attribute.
Patch by chris.wailes@gmail.com

Functions can now declare what state the consumable type the are returning will
be in. This is then used on the caller side and checked on the callee side.
Constructors now use this attribute instead of the 'consumes' attribute.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@189843 91177308-0d34-0410-b5e6-96231b3b80d8