[Syntax] Implement return-statement and integer-literal-expr

A return statement needs something to return, so implement
integer-literal-expression too. This necessarily also forced
UnknownExprSyntax, UnknownStmtSyntax, and UnknownDeclSyntax,
which are stand-in token buckets for when we don't know
how to transform/migrate an AST.

This commit also contains the core function for caching
SyntaxData children. This is highly tricky code, with some
detailed comments in SyntaxData.{h,cpp}. The gist is that
we have to atomically swap in a SyntaxData pointer into the
child field, so we can maintain pointer identity of SyntaxData
nodes, while still being able to cache them internally.

To prove that this works, there is a multithreaded test that
checks that two threads can ask for a child that hasn't been
cached yet without crashing or violating pointer identity.

https://bugs.swift.org/browse/SR-4010

lib/Syntax/DeclSyntax.cpp

@@ -31,6 +31,32 @@ DeclSyntaxData::DeclSyntaxData(RC<RawSyntax> Raw,
 DeclSyntax::DeclSyntax(const RC<SyntaxData> Root, const DeclSyntaxData *Data)
   : Syntax(Root, Data) {}
 
+#pragma mark - unknown-statement Data
+
+UnknownDeclSyntaxData::UnknownDeclSyntaxData(RC<RawSyntax> Raw,
+                                             const SyntaxData *Parent,
+                                             CursorIndex IndexInParent)
+  : DeclSyntaxData(Raw, Parent, IndexInParent) {
+  assert(Raw->Kind == SyntaxKind::UnknownStmt);
+}
+
+RC<UnknownDeclSyntaxData>
+UnknownDeclSyntaxData::make(RC<RawSyntax> Raw,
+                            const SyntaxData *Parent,
+                            CursorIndex IndexInParent) {
+  return RC<UnknownDeclSyntaxData> {
+    new UnknownDeclSyntaxData {
+      Raw, Parent, IndexInParent
+    }
+  };
+}
+
+#pragma mark - unknown-statement API
+
+UnknownDeclSyntax::UnknownDeclSyntax(const RC<SyntaxData> Root,
+                                     const UnknownDeclSyntaxData *Data)
+  : DeclSyntax(Root, Data) {}
+
 #pragma mark - declaration-members Data
 
 DeclMembersSyntaxData::DeclMembersSyntaxData(RC<RawSyntax> Raw,