Sema: discard self should not be allowed in inlinable methods of non-frozen types.

`discard self` requires knowledge of the internal layout of the type in order to clean
up its fields while bypassing its public `deinit`. Inlinable code can get copied into
and run from outside of the defining module, so this is impossible to implement.
Fixes rdar://160815058.

test/ModuleInterface/discard_interface.swift

@@ -8,6 +8,7 @@
 // CHECK: @_alwaysEmitIntoClient public consuming func AEIC_discard() { discard self }
 // CHECK: @inlinable public consuming func inlinable_discard() { discard self }
 
+@frozen
 public struct MoveOnlyStruct: ~Copyable {
   let x = 0