Add an option `-enable-old-style-attr-syntax' to print out function attributes in the "old" style.

It's sometimes beneficial to emit a testcase with the old style attribute
syntax. Allow someone to do this.
<rdar://problem/13563209>


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@179735 91177308-0d34-0410-b5e6-96231b3b80d8