[TI removal] Update the C API for the move away from `TerminatorInst`.

This updates the C API for the removal of `TerminatorInst`. It converts
the type query to a predicate query and moves the generic methods to
work on `Instruction` instances that satisfy this predicate rather than
requiring a specific type. It also clarifies that the C API wrapping
`BasicBlock::getTerminator` just returns an `Instruction`. Because this
was always wrapped opaquely as a value and the functions consuming these
values will work on `Instruction` objects, this shouldn't break any
clients.

This is a completely compatible change to the C API.

Differential Revision: https://reviews.llvm.org/D52968

llvm-svn: 344764