mat-icon has display issues when inlining multiple copies of the same SVG with defs with IDs

#### Bug, feature request, or proposal:
Proposal: Extend the cloneSvg function in src/lib/icon/icon-registry.ts to accommodate munging IDs in defs so that multiple copies of the same inlined SVG won't have conflicting IDs.

#### What is the expected behavior?
When two copies of an SVG with a linearGradient def (identified by an ID) are inlined, both copies of the SVG should display the linear gradient.

#### What is the current behavior?
Only the first copy of the SVG displays the linear gradient. All other copies do not, because the IDs in the defs are duplicated in the other copies.

#### Which versions of Angular, Material, OS, TypeScript, browsers are affected?
Angular 5.1.2, Angular Material 5.0.2 (although the issue still exists in 5.1.0), TypeScript 2.6.2. Tested in Win10 in Chrome Version 63.0.3239.132 (Official Build) (64-bit).

Empieza en src/lib/icon/icon-registry.ts, en cloneSvg, e inspecciona cómo se copian los defs de SVG y sus IDs. Reproduce el problema con dos copias inline que contengan un linearGradient y verifica después que cada copia renderice su gradiente sin conflictos de ID.

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