Assert immediately if someone tries to build a SILFunctionType with

multiple isolated parameters.

Author: rjmccall

lib/AST/ASTContext.cpp

@@ -5350,8 +5350,13 @@ SILFunctionType::SILFunctionType(
              "Cannot return an @noescape function type");
     }
   }
+  bool hasIsolatedParameter = false; (void) hasIsolatedParameter;
   for (auto param : getParameters()) {
-    (void)param;
+    if (param.hasOption(SILParameterInfo::Isolated)) {
+      assert(!hasIsolatedParameter &&
+             "building SIL function type with multiple isolated parameters");
+      hasIsolatedParameter = true;
+    }
     assert(!isa<PackExpansionType>(param.getInterfaceType()) &&
            "Cannot have a pack expansion directly as a parameter");
     assert(param.isPack() == isa<SILPackType>(param.getInterfaceType()) &&