[no-implicit-copy] Assert in SILBuilder that mark_must_check only can take a MoveOnlyWrapped value.

In the future, we may not have this requirement, but for now it is a really
convenient way to track down places in SILGen where I have missed converting
from copyable to moveonlywrapped types.

Author: gottesmm

include/swift/SIL/SILInstruction.h

@@ -7580,7 +7580,10 @@ class MarkMustCheckInst
       : UnaryInstructionBase(DebugLoc, operand, operand->getType()),
         OwnershipForwardingMixin(SILInstructionKind::MarkMustCheckInst,
                                  operand->getOwnershipKind()),
-        kind(checkKind) {}
+        kind(checkKind) {
+    assert(operand->getType().isMoveOnlyWrapped() &&
+           "mark_must_check can only take a move only wrapped value");
+  }
 
 public:
   CheckKind getCheckKind() const { return kind; }