Distinguish pattern bound symbol vs. case class flag usage

[milessabin]

Follow on from #6172.

See comment by @smarter here.

See also, https://github.com/lampepfl/dotty/blob/7bcaa97bd98aa2e039fb90aba81c3fde98ec65fe/compiler/src/dotty/tools/dotc/transform/PatternMatcher.scala#L88-L91

Which suggests that maybe we shouldn't be using Case here at all?

[milessabin]

I had a quick look at using isPatmatGenerated in the inliner (where the missing unpickled Case caused #6157) but that doesn't work because the symbol name doesn't conform to the expected pattern. I don't know whether or not this means TODO quoted above can be resolved.

[abgruszecki]

@milessabin I'm uncertain if using isPatmatGenerated in the inliner makes sense - there, we're looking for results of newPatternBoundSymbol, which only sets Case flag. newPatternBoundSymbol also seems to be the counterpart of createBindSymbol, which you needed to adjust in #6172.

[liufengyun]

#7036 Fixes and checks flags of pattern bound symbols:

    /** Is the symbol a pattern bound symbol?
     */
    final def isPatternBound(implicit ctx: Context): Boolean =
      !isClass && this.is(Case, butNot = Enum | Module)