Tweak -Xprint output

[dwijnand]

Most importantly, move "unchanged since $phase" to be on the same line
as the lead-in. And then I copied over the lead-in from Scala 2,
because I like it. It doesn't look as nice because MegaPhases don't
have their own trim name, but it starts nicely:

[[syntax trees at end of                     typer]] // a.scala
package <empty> {
  class C() extends Object() {
    def foo: Int = 1
  }
}

[[syntax trees at end of          inlinedPositions]] // a.scala: unchanged since typer
[[syntax trees at end of                 posttyper]] // a.scala
package <empty> {
  @scala.annotation.internal.SourceFile("a.scala") class C() extends Object() {
    def foo: Int = 1
  }
}

[[syntax trees at end of                   pickler]] // a.scala: unchanged since posttyper
[[syntax trees at end of                  inlining]] // a.scala: unchanged since posttyper
[[syntax trees at end of              postInlining]] // a.scala: unchanged since posttyper
[[syntax trees at end of                   staging]] // a.scala: unchanged since posttyper
[[syntax trees at end of              pickleQuotes]] // a.scala: unchanged since posttyper
[[syntax trees at end of MegaPhase{firstTransform, checkReentrant, elimPackagePrefixes, cookComments, checkStatic, betaReduce, inlineVals, expandSAMs}]] // a.scala: unchanged since posttyper
[[syntax trees at end of MegaPhase{elimRepeated, protectedAccessors, extmethods, uncacheGivenAliases, byNameClosures, hoistSuperArgs, specializeApplyMethods, refchecks, tryCatchPatterns, patternMatcher}]] // a.scala: unchanged since posttyper
[[syntax trees at end of MegaPhase{elimOpaque, explicitOuter, explicitSelf, elimByName, stringInterpolatorOpt}]] // a.scala: unchanged since posttyper
[[syntax trees at end of MegaPhase{pruneErasedDefs, uninitializedDefs, inlinePatterns, vcInlineMethods, seqLiterals, intercepted, getters, specializeFunctions, liftTry, collectNullableFields, elimOuterSelect, resolveSuper, functionXXLForwarders, paramForwarding, genericTuples, letOverApply, arrayConstructors}]] // a.scala: unchanged since posttyper

[nicolasstucki]

Otherwise LGTM