[IR] Add operand bundles to CallInst and InvokeInst.

Summary:
This change teaches `CallInst`s and `InvokeInst`s to maintain a set of
operand bundles as part of its operands.  `CallInst`s and `InvokeInst`s
with operand bundles co-allocate some space before their `Use` array to
hold meta information about which of its operands are part of an operand
bundle.

The strings corresponding to the bundle tags are interned into
`LLVMContextImpl::BundleTagCache`

This change does not include any parsing / bitcode support.  That's the
next change.

Depends on D12455.

Reviewers: reames, chandlerc, majnemer, dexonsmith, kmod, JosephTremoulet, rnk, bogner

Subscribers: MatzeB, sanjoy, llvm-commits

Differential Revision: http://reviews.llvm.org/D12456

llvm-svn: 248527