go/types: remove MethodSetCache - not needed

In x/tools, MethodSetCache was moved from x/tools/go/types to
x/tools/go/types/typeutil. Mirror that change.

Change-Id: Ib838a9518371473c83fa4abc2778d42f33947c98
Reviewed-on: https://go-review.googlesource.com/10771Reviewed-by: Alan Donovan <adonovan@google.com>

src/go/types/methodsetcache.go

-// Copyright 2014 The Go Authors. All rights reserved.
-// Use of this source code is governed by a BSD-style
-// license that can be found in the LICENSE file.
-
-// This file implements a cache of method sets.
-
-package types
-
-import "sync"
-
-// A MethodSetCache records the method set of each type T for which
-// MethodSet(T) is called so that repeat queries are fast.
-// The zero value is a ready-to-use cache instance.
-type MethodSetCache struct {
-	mu     sync.Mutex
-	named  map[*Named]struct{ value, pointer *MethodSet } // method sets for named N and *N
-	others map[Type]*MethodSet                            // all other types
-}
-
-// MethodSet returns the method set of type T.  It is thread-safe.
-//
-// If cache is nil, this function is equivalent to NewMethodSet(T).
-// Utility functions can thus expose an optional *MethodSetCache
-// parameter to clients that care about performance.
-//
-func (cache *MethodSetCache) MethodSet(T Type) *MethodSet {
-	if cache == nil {
-		return NewMethodSet(T)
-	}
-	cache.mu.Lock()
-	defer cache.mu.Unlock()
-
-	switch T := T.(type) {
-	case *Named:
-		return cache.lookupNamed(T).value
-
-	case *Pointer:
-		if N, ok := T.Elem().(*Named); ok {
-			return cache.lookupNamed(N).pointer
-		}
-	}
-
-	// all other types
-	// (The map uses pointer equivalence, not type identity.)
-	mset := cache.others[T]
-	if mset == nil {
-		mset = NewMethodSet(T)
-		if cache.others == nil {
-			cache.others = make(map[Type]*MethodSet)
-		}
-		cache.others[T] = mset
-	}
-	return mset
-}
-
-func (cache *MethodSetCache) lookupNamed(named *Named) struct{ value, pointer *MethodSet } {
-	if cache.named == nil {
-		cache.named = make(map[*Named]struct{ value, pointer *MethodSet })
-	}
-	// Avoid recomputing mset(*T) for each distinct Pointer
-	// instance whose underlying type is a named type.
-	msets, ok := cache.named[named]
-	if !ok {
-		msets.value = NewMethodSet(named)
-		msets.pointer = NewMethodSet(NewPointer(named))
-		cache.named[named] = msets
-	}
-	return msets
-}