[AST] Remove the "single expression body" bit

Remove this bit from function decls and closures.
Instead, for closures, infer it from the presence
of a single return or single expression AST node
in the body, which ought to be equivalent, and
automatically takes result builders into
consideration. We can also completely drop this
query from AbstractFunctionDecl, replacing it
instead with a bit on ReturnStmt.

lib/Sema/DebuggerTestingTransform.cpp

@@ -279,7 +279,7 @@ private:
     ASTNode ClosureElements[] = {OriginalExpr, CheckExpectExpr};
     auto *ClosureBody = BraceStmt::create(Ctx, SourceLoc(), ClosureElements,
                                           SourceLoc(), /*Implicit=*/true);
-    Closure->setBody(ClosureBody, /*isSingleExpression=*/false);
+    Closure->setBody(ClosureBody);
 
     // Call the closure.
     auto *ClosureCall = CallExpr::createImplicitEmpty(Ctx, Closure);