swift234

Add an argument-number field to DebugValueInst and friends.
This commit adds a DebugVariable field that is shared by
- AllocBoxInst
- AllocStackInst
- DebugValueInst
- DebugValueAddrInst
Currently DebugVariable only holds the Swift argument number.

This allows us to retire several expensive heuristics in IRGen that
attempted to identify which local variables actually where arguments
and recover their relative order.

Memory footprint notes:
This commit adds a 4-byte field to 4 SILInstructin subclasses.
This was offset by 8ab1e2dd502fc7f37a67572699165e03f268c377
which removed 20 bytes from *every* SILInstruction.

Caveats:
This commit surfaces a known bug in FunctionSigantureOpts, tracked in
rdar://problem/23727705 — debug info for exploded function arguments
cannot be expressed until this is fixed.

<rdar://problem/21185379&22705926>