[IR] Add a `data_operand` abstraction

Summary:
Data operands of a call or invoke consist of the call arguments, and
the bundle operands associated with the `call` (or `invoke`)
instruction.  The motivation for this change is that we'd like to be
able to query "argument attributes" like `readonly` and `nocapture`
for bundle operands naturally.

This change also provides a conservative "implementation" for these
attributes for any bundle operand, and an extension point for future
work.

Reviewers: chandlerc, majnemer, reames

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D14305

llvm-svn: 252077