fix: add computed property support for object Ref

[JLHwung]

Q	A
Fixed Issues?	Fixes #6341 (comment)
Patch: Bug Fix?	Yes
Tests Added + Pass?	Yes
License	MIT

In this PR we add support for computed properties when generating objRef for object-rest-spread. In the following example

var prop = "a";
var obj = { a: { b: 42 } };
var {
  [prop]: { ...x }
} = obj;

// transformed on v7.7.5

var prop = "a";
var obj = {
  a: {
    b: 42
  }
};

var {} = obj,
    x = _extends({}, obj.prop);

Here the objRef is obj.prop, which is the parent path of the rest element ...x. Apparently obj.prop is incorrect because [prop] is a computed property. This is fixed by adding computed argument when building t.memberExpression for objRef.

However, the computed properties could be impure: which means we could not evaluate these properties twice. Therefore we refactored the interface of replaceImpureComputedKeys and reuse it to cache the computed properties.

Note that although this PR only addresses #6341 (comment), #6341 can also be closed after merging this PR because I can not reproduce OP's issue on REPL. (link)

[JLHwung]

This test case is revised from

babel/packages/babel-plugin-proposal-object-rest-spread/test/fixtures/object-rest/impure-computed/input.js

Line 1 in 0b3f883

var key, x, y, z;

[nicolo-ribaudo]

The failing tests are fixed in master