Verify SILSelf parameter type when we create the function in SILModule::getOrCreateFunction instead of moving it into the verifier.

This allows me to remove the map I added to SILModule.

Swift SVN r27242

lib/SIL/SILFunction.cpp

@@ -455,7 +455,3 @@ ArrayRef<Substitution> SILFunction::getForwardingSubstitutions() {
     return {};
   return params->getForwardingSubstitutions(getASTContext());
 }
-
-llvm::Optional<SILDeclRef> SILFunction::getDeclRef() const {
-  return getModule().lookUpDeclRef(this);
-}