Teach the SIL verifier to enforce stronger rules around

edges into dead-end regions.

- Only treat edges *into* dead-end regions as special; edges internal
  the region must use the normal rules.

- Conservatively merge information along those edges rather than just
  picking one at random. This requires us to not walk into the region
  until we've processed all of the edges to it.

- Make sure we prevent *any* stack allocations from being deallocated
  if the stack is inconsistent entering a block.

Additionally, fix a bug which was incorrectly treating all blocks that
don't themselves exit the function as ultimately leading to unreachable,
which had inadvertently largely turned off the consistency check.

Author: rjmccall

include/swift/SIL/SILModule.h

@@ -46,6 +46,7 @@
 #include "llvm/ADT/MapVector.h"
 #include "llvm/ADT/PointerIntPair.h"
 #include "llvm/ADT/SetVector.h"
+#include "llvm/ADT/STLFunctionalExtras.h"
 #include "llvm/ADT/ilist.h"
 #include "llvm/ProfileData/InstrProfReader.h"
 #include "llvm/Support/Allocator.h"
@@ -1144,7 +1145,7 @@ inline llvm::raw_ostream &operator<<(llvm::raw_ostream &OS, const SILModule &M){
 void verificationFailure(const Twine &complaint,
               const SILInstruction *atInstruction,
               const SILArgument *atArgument,
-              const std::function<void()> &extraContext);
+              llvm::function_ref<void(llvm::raw_ostream &OS)> extraContext);
 
 inline bool SILOptions::supportsLexicalLifetimes(const SILModule &mod) const {
   switch (mod.getStage()) {