Create calling convention AttributedType sugar nodes

Canonical types are unchanged.  The type printer had to be changed to
avoid printing any non-default implicit calling convention as well as
the calling convention attribute.

Reviewers: rjmccall

Differential Revision: http://llvm-reviews.chandlerc.com/D1132

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@186714 91177308-0d34-0410-b5e6-96231b3b80d8