Revert "Fix callback types to match covariantly"

This reverts commit eebd67f.

Author: mhegazy

src/compiler/parser.ts

@@ -57,7 +57,7 @@ namespace ts {
         // The visitXXX functions could be written as local functions that close over the cbNode and cbNodeArray
         // callback parameters, but that causes a closure allocation for each invocation with noticeable effects
         // on performance.
-        const visitNodes: (cb: ((node: Node) => T) | ((node: Node[]) => T), nodes: Node[]) => T = cbNodeArray ? visitNodeArray : visitEachNode;
+        const visitNodes: (cb: (node: Node | Node[]) => T, nodes: Node[]) => T = cbNodeArray ? visitNodeArray : visitEachNode;
         const cbNodes = cbNodeArray || cbNode;
         switch (node.kind) {
             case SyntaxKind.QualifiedName:

src/compiler/visitor.ts

@@ -885,7 +885,7 @@ namespace ts {
             return initial;
         }
 
-        const reduceNodes: (nodes: NodeArray<Node>, f: ((memo: T, node: Node) => T) | ((memo: T, node: NodeArray<Node>) => T), initial: T) => T = cbNodeArray ? reduceNodeArray : reduceLeft;
+        const reduceNodes: (nodes: NodeArray<Node>, f: (memo: T, node: Node | NodeArray<Node>) => T, initial: T) => T = cbNodeArray ? reduceNodeArray : reduceLeft;
         const cbNodes = cbNodeArray || cbNode;
         const kind = node.kind;