Rewrite SimpleDidSetRequest to not require a typechecked body

SimpleDidSetRequest's implementation was depending on type-checking
the body of the observer, meaning that computing the interface type of
a `didSet` would require type-checking the body first. This triggers
some request cycles, and is unnecessarily complicated.

Rewrite the implementation to work on a non-type-checked body, by
using name lookup to establish when the `oldValue` parameter is
referenced from the body.

Author: DougGregor

lib/AST/Decl.cpp

@@ -957,14 +957,6 @@ 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>())

lib/Sema/TypeCheckStorage.cpp

@@ -3797,6 +3797,9 @@ bool SimpleDidSetRequest::evaluate(Evaluator &evaluator,
     virtual PreWalkResult<Expr *> walkToExprPre(Expr *E) override {
       if (!E)
         return Action::Continue(E);
+
+      // If we have an already-resolved reference to the parameter in question,
+      // we're done.
       if (auto DRE = dyn_cast<DeclRefExpr>(E)) {
         if (auto decl = DRE->getDecl()) {
           if (decl == OldValueParam) {
@@ -3806,6 +3809,20 @@ bool SimpleDidSetRequest::evaluate(Evaluator &evaluator,
         }
       }
 
+      // If we have an unresolved reference with the same name as the parameter,
+      // perform name lookup to determine whether it refers to this parameter.
+      if (auto UDRE = dyn_cast<UnresolvedDeclRefExpr>(E)) {
+        auto loc = UDRE->getLoc();
+        auto sf = OldValueParam->getDeclContext()
+            ->getOutermostParentSourceFile();
+        auto name = UDRE->getName().getFullName();
+        if (name == OldValueParam->getName() &&
+            ASTScope::lookupSingleLocalDecl(sf, name, loc) == OldValueParam) {
+          foundOldValueRef = true;
+          return Action::Stop();
+        }
+      }
+
       return Action::Continue(E);
     }
 
@@ -3838,7 +3855,7 @@ bool SimpleDidSetRequest::evaluate(Evaluator &evaluator,
   // If we find a reference to the implicit 'oldValue' parameter, then it is
   // not a "simple" didSet because we need to fetch it.
   auto walker = OldValueFinder(param);
-  if (auto *body = decl->getTypecheckedBody())
+  if (auto *body = decl->getBody())
     body->walk(walker);
   auto hasOldValueRef = walker.didFindOldValueRef();
   if (!hasOldValueRef) {