[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

Comienza en el punto de entrada de posicionamiento de Angular Material mat-menu y sigue cómo overlapTrigger gestiona actualmente el eje y frente a xPosition. Ejecuta las pruebas relevantes del menú y, después, verifica que establecer overlapTrigger en false evita la superposición horizontal con el elemento desencadenante, conservando el comportamiento existente cuando es true.

Escrito por el modelo de indexación a partir del texto del issue.