Analyzer warnings for unused variables

[blois]

void foo() {
 // Should be a warning- it's assigned but never referenced.
 var something = 1;
}

This came up in a bug, it's something which I've felt was helpful from other analyzers.

[bwilkerson]

Added this to the Later milestone.
Removed Type-Defect label.
Added Type-Enhancement label.

[clayberg]

Removed this from the Later milestone.
Added this to the M4 milestone.
Added Duplicate label.
Marked as being merged into #2463.