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).

src/lib/icon/icon-registry.ts の cloneSvg から始め、SVG の defs とその ID がどのようにコピーされるかを調べます。linearGradient を含むインライン化されたコピーを 2 つ使って問題を再現し、その後、各コピーが ID の競合なしにグラデーションをレンダリングすることを確認します。

索引モデルが issue の本文から書いたものです。