Preserve local variables names in NIR

[WojciechMazur]

Enriches Defn.Define with additional field localNames: Map[Local, String] containing original name of local variables (both val and var) allowing us to emit additional debug informations in the future.

The following snippet now compiles to NIR containing local names:

  def main(args: Array[String]): Unit = {
    val x = 42
    val y = x
    var z = 0
    z = x + y

    val a = "Foo".toLowerCase()
  }

%3(%2 <this> : @"T5Test$", %1 <args> : array[@"T16java.lang.String"]):
  %4 <z> = var[int]
  %5 <x> = copy int 42
  %6 <y> = copy %5 <x> : int
  %7 = varstore %4 <z> : var[int], int 0
  %8 = iadd[int] %5 <x> : int, %6 <y> : int
  %9 = varstore %4 <z> : var[int], %8 : int
  %10 = method "Foo", "D11toLowerCaseL16java.lang.StringEO"
  %11 = call[(@"T16java.lang.String") => @"T16java.lang.String"] %10 : ptr("Foo")
  %12 <a> = copy %11 : @"T16java.lang.String"
  ret unit
}

The literal values are now assigned to local variables (e.g. x, y) to allow for correct mapping of them in debug metadata. The copy operator is effectivelly no-op. It could only affect Val.StructValue and Val.ArrayValue which currently cannot be assigned to variables (low level primitives)

The size of binaries might slightly increase. Scalalib 2.13 now takes 24,104 kB vs 23,910 kB previously (when compared to last NIR encoding changes in #3346)

TODO:

• Preserve name information in the optimizer transformations
• Preserve name information in the lowering transformations