SIL: introduce a PlaceholderValue and use it in the parser and deserializer.

... instead of a GlobalAddrInst.
This is cleaner and makes the handling of forward-referenced values in the deserializer a bit simpler.

lib/SIL/IR/SILModule.cpp

@@ -169,6 +169,9 @@ void SILModule::checkForLeaks() const {
     llvm::errs() << "Instructions in module: " << instsInModule << '\n';
     llvm_unreachable("leaking instructions");
   }
+  
+  assert(PlaceholderValue::getNumPlaceholderValuesAlive() == 0 &&
+         "leaking placeholders");
 }
 
 void SILModule::checkForLeaksAfterDestruction() {