Placeholder overflow is hidden now

[JunusErgin]

If the placeholder is very long, the overflow should be hidden and replaced with three dots instead.
Otherwise, a long placeholder will be displayed in two lines.

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[JunusErgin]

I signed it!

[googlebot]

CLAs look good, thanks!

[mmalerba]

This shouldn't be necessary, #6488 will shift responsibility for managing the placeholder to md-form-field which already has the correct behavior

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