continuation prior to preemption

Right now, basically zero uses of something of a move-only type are allowed.

Plan at this point was to go through CSGen.cpp looking for all calls to
ConstraintSystem::createTypeVariable and manually determine whether we
should allow a move-only type to be bound to that variable.

Places I already expect to not allow it are variables generated for
optional-unwrapping expressions, since they're not allowed to appear
in optionals for now.

What I planned to do was have one expression / case cover each kind of
type variable as I go through and add TVO_CanBindToMoveOnly to the options flag set.

Author: kavon

test/Constraints/move-only-ban-generics.swift

@@ -0,0 +1,20 @@
+// RUN: %target-typecheck-verify-swift -enable-experimental-move-only
+
+// a concrete move-only type
+@_moveOnly struct MO {
+  var x: Int?
+}
+
+@_moveOnly struct Container {
+  var mo: MO = MO()
+}
+
+// utilities
+
+func takeConcrete(_ m: MO) {}
+func takeGeneric<T>(_ t: T) {}
+
+// actual tests
+
+//var globalMO: MO = MO()
+//let _: MO = globalMO