Allow underscore for variable name in interpolated string in pattern

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Allow patterns like s"$\_" and s"${_}".

It would be nice if this worked:

val q"$foo(..$_)" = x

How many distinct uses of underscore does this make?

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Imported From: https://issues.scala-lang.org/browse/SI-7715?orig=1
Reporter: @som-snytt
Affected Versions: 2.11.0-M4

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@som-snytt said:
Limit scope.

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@adriaanm said:
scala/scala#2823

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@som-snytt said:
Vox populi http://stackoverflow.com/questions/29191294/use-in-map-for-string-interpolation