[Typed throws] Location based lookup for the thrown error type

Introduce a new API to find the AST node that catches or rethrows an
error thrown from the given source location. Use it to determine the
thrown error type to use for type checking a `throw` statement, which
begins as `any Error` within a `do..catch` and is later refined.

lib/AST/Decl.cpp

@@ -11398,3 +11398,32 @@ MacroDiscriminatorContext::getParentOf(FreestandingMacroExpansion *expansion) {
   return getParentOf(
       expansion->getPoundLoc(), expansion->getDeclContext());
 }
+
+llvm::Optional<Type>
+CatchNode::getThrownErrorTypeInContext(ASTContext &ctx) const {
+  if (auto func = dyn_cast<AbstractFunctionDecl *>()) {
+    if (auto thrownError = func->getEffectiveThrownErrorType())
+      return func->mapTypeIntoContext(*thrownError);
+
+    return llvm::None;
+  }
+
+  if (auto closure = dyn_cast<AbstractClosureExpr *>()) {
+    if (closure->getType())
+      return closure->getEffectiveThrownType();
+
+    // FIXME: Should we lazily compute this?
+    return llvm::None;
+  }
+
+  auto doCatch = get<DoCatchStmt *>();
+  if (auto thrownError = doCatch->getCaughtErrorType()) {
+    if (thrownError->isNever())
+      return llvm::None;
+
+    return thrownError;
+  }
+
+  // If we haven't computed the error type yet, do so now.
+  return ctx.getErrorExistentialType();
+}