[Typed throws] Parsing and AST representation for typed errors

Parse typed throw specifiers as `throws(X)` in every place where there
are effects specified, and record the resulting thrown error type in
the AST except the type system. This includes:
* `FunctionTypeRepr`, for the parsed representation of types
* `AbstractFunctionDecl`, for various function-like declarations
* `ClosureExpr`, for closures
* `ArrowExpr`, for parsing of types within expression context

This also introduces some serialization logic for the thrown error
type of function-like declarations, along with an API to extract the
thrown interface type from one of those declarations, although right
now it will either be `Error` or empty.

lib/Sema/DerivedConformanceDistributedActor.cpp

@@ -108,6 +108,7 @@ static FuncDecl *deriveDistributedActor_resolve(DerivedConformance &derived) {
                                name, SourceLoc(),
                                /*async=*/false,
                                /*throws=*/true,
+                               /*ThrownType=*/Type(),
                                /*genericParams=*/nullptr,
                                params,
                                /*returnType*/decl->getDeclaredInterfaceType(),
@@ -257,7 +258,9 @@ static FuncDecl* createLocalFunc_doInvokeOnReturn(
       DeclName(C, doInvokeLocalFuncIdent, doInvokeParamsList),
       sloc,
       /*async=*/true,
-      /*throws=*/true, doInvokeGenericParamList, doInvokeParamsList,
+      /*throws=*/true, 
+      /*ThrownType=*/Type(),
+      doInvokeGenericParamList, doInvokeParamsList,
       /*returnType=*/C.TheEmptyTupleType, parentFunc);
   doInvokeOnReturnFunc->setImplicit();
   doInvokeOnReturnFunc->setSynthesized();
@@ -419,6 +422,7 @@ static FuncDecl *deriveDistributedActorSystem_invokeHandlerOnReturn(
       FuncDecl::createImplicit(C, StaticSpellingKind::None, name, SourceLoc(),
                                /*async=*/true,
                                /*throws=*/true,
+                               /*ThrownType=*/Type(),
                                /*genericParams=*/nullptr, params,
                                /*returnType*/ TupleType::getEmpty(C), system);
   funcDecl->setSynthesized(true);