Currently string attributes on function arguments/return values can be generated...

Currently string attributes on function arguments/return values can be generated using LLVM API. However they are not supported in parser. So, the following scenario will fail:
* generate function with string attribute using API,
* dump it in LL format,
* try to parse.
Add parser support for string attributes to fix the issue.

Reviewed By: reames, hfinkel

Differential Revision: http://reviews.llvm.org/D11058

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@243877 91177308-0d34-0410-b5e6-96231b3b80d8