[silgen] Fix up EnumElementPatternInitialization::emitEnumMatch to use ownership.

This commit does a few things:

1. It uses SwitchEnumBuilder so we are not re-inventing any wheels.
2. Instead of hacking around not putting in a destroy for .None on the fail
pass, just *do the right thing* and recognize that we have a binary case enum
and in such a case, just emit code for the other case rather than use a default
case (meaning no cleanup on .none).

rdar://31145255

lib/SIL/SILBasicBlock.cpp

@@ -346,3 +346,10 @@ bool SILBasicBlock::isNoReturn() const {
 
   return false;
 }
+
+bool SILBasicBlock::isTrampoline() const {
+  auto *Branch = dyn_cast<BranchInst>(getTerminator());
+  if (!Branch)
+    return false;
+  return begin() == Branch->getIterator();
+}