IR: Add BasicBlock::insertInto()

Although unlinked `BasicBlock`s can be created, there's currently no way
to insert them into `Function`s after the fact.  In particular,
`moveAfter()` and `moveBefore()` require that the basic block is already
linked.

Extract the logic for initially linking a `BasicBlock` out of the
constructor and into a member function that can be used for lazy
insertion.

  - Asserts that the basic block is currently unlinked.
  - Matches the logic of the constructor.
  - Changed the constructor to use it since the logic matches.

This is needed in a follow-up commit for PR5680.

llvm-svn: 214563