Enable a compiler plugin to use the async transform after patmat

[retronym]

Currently, the async transformation is performed during the typer
phase, like all other macros.

We have to levy a few artificial restrictions on whern an async
boundary may be: for instance we don't support await within a
pattern guard. A more natural home for the transform would be
after patterns have been translated.

The test case in this commit shows how to use the async transform
from a custom compiler phase after patmat.

The remainder of the commit updates the implementation to handle
the new tree shapes.

For states that correspond to a label definition, we use -symbol.id
as the state ID. This made it easier to emit the forward jumps to when
processing the label application before we had seen the label
definition.

I've also made the transformation more efficient in the way it checks
whether a given tree encloses an await call: we traverse the input
tree at the start of the macro, and decorate it with tree attachments
containig the answer to this question. Even after the ANF and state
machine transforms introduce new layers of synthetic trees, the
containsAwait code need only traverse shallowly through those
trees to find a child that has the cached answer from the original
traversal.

I had to special case the ANF transform for expressions that always
lead to a label jump: we avoids trying to push an assignment to a result
variable into if (cond) jump1() else jump2(), in trees of the form:

% cat sandbox/jump.scala
class Test {
  def test = {
    (null: Any) match {
      case _: String => ""
      case _ => ""
    }
  }
}

% qscalac -Xprint:patmat -Xprint-types sandbox/jump.scala
    def test: String = {
      case <synthetic> val x1: Any = (null{Null(null)}: Any){Any};
      case5(){
        if (x1.isInstanceOf{[T0]=> Boolean}[String]{Boolean})
          matchEnd4{(x: String)String}(""{String("")}){String}
        else
          case6{()String}(){String}{String}
      }{String};
      case6(){
        matchEnd4{(x: String)String}(""{String("")}){String}
      }{String};
      matchEnd4(x: String){
        x{String}
      }{String}
    }{String}

[lrytz]

maybe a bit of clarification here

• the method name suggests that it eliminates multiple label parameters, but it expects there to be one
• quickly recap the structure generated by patmat. is the following correct?
  • normal case defs are label defs without parameters.
  • the single matchEnd label def has excatly one (zero or one? it seems to have one also for unit-typed matches) parameter.

[lrytz]

great work, looks very good!

[retronym]

Thanks for the review, @lrytz. I've incorporated your suggestions.