[mat-menu] support horizontal (xPosition) overlap detection

#### Bug, feature request, or proposal:
feature

#### What is the expected behavior?
When [overlapTrigger] is false the menu will NOT overlap on the horizontal axis (xPosition)

#### What is the current behavior?
The horizontal axis is overlapping the triggering element wether overlapTrigger is true/false.

#### What is the use-case or motivation for changing an existing behavior?
Per the docs, the overlapTrigger should respect both x and y.
It is also a valid requirement.

#### Which versions of Angular, Material, OS, TypeScript, browsers are affected?
angular 5, material 5.0.0-rc0

从 Angular Material mat-menu 的定位入口开始，跟踪 overlapTrigger 当前如何处理 y 轴与 xPosition 的关系。运行相关的菜单测试，然后验证将 overlapTrigger 设置为 false 会阻止与触发元素发生水平重叠，同时在其为 true 时保留现有行为。

由索引模型根据 Issue 内容生成。