Untangle the mess which is MachineBasicBlock::hasAddressTaken().

There are two different senses in which a block can be "address-taken".
There can be a BlockAddress involved, which means we need to map the
IR-level value to some specific block of machine code.  Or there can be
constructs inside a function which involve using the address of a basic
block to implement certain kinds of control flow.

Mixing these together causes a problem: if target-specific passes are
marking random blocks "address-taken", if we have a BlockAddress, we
can't actually tell which MachineBasicBlock corresponds to the
BlockAddress.

So split this into two separate bits: one for BlockAddress, and one for
the machine-specific bits.

Discovered while trying to sort out related stuff on D102817.

Differential Revision: https://reviews.llvm.org/D124697