[Typed throws] Avoid dependency cycle between SimpleDidSetRequest and interface types

The implementation of `SimpleDidSetRequest` currently depends on a
type-checked body for `didSet`, which in turn depends on the interface
type (for the thrown error type). Break that dependency in a silly way
for now.

Author: DougGregor

lib/AST/Decl.cpp

@@ -957,6 +957,14 @@ Type AbstractFunctionDecl::getThrownInterfaceType() const {
 
 llvm::Optional<Type> 
 AbstractFunctionDecl::getEffectiveThrownErrorType() const {
+  // FIXME: Only getters can have thrown error types right now, and DidSet
+  // has a cyclic reference if we try to get its interface type here. Find a
+  // better way to express this.
+  if (auto accessor = dyn_cast<AccessorDecl>(this)) {
+    if (accessor->getAccessorKind() != AccessorKind::Get)
+      return llvm::None;
+  }
+
   Type interfaceType = getInterfaceType();
   if (hasImplicitSelfDecl()) {
     if (auto fnType = interfaceType->getAs<AnyFunctionType>())