`$outer` is not marked synthetic

[raboof]

I suspect it should be, right? We're now explicitly excluding it in spray-json (https://github.com/spray/spray-json/pull/342/files#diff-02c1dfdd5a87b7d8a1793e8a95fd5ea6467c9b55c1cd942402d84ea474ee4aaaR77)

Compiler version

Any, including 3.1.0 and 3.1.2-RC1-bin-20211208-d6fe4b4-NIGHTLY

Minimized code

class Outer {
  class Inner
}

Output

$ javap -v Outer\$Inner.class
(...)
  public final Outer Outer$Inner$$$outer();
    descriptor: ()LOuter;
    flags: (0x0011) ACC_PUBLIC, ACC_FINAL

Expectation

$ javap -v Outer\$Inner.class
(...)
  public final Outer Outer$Inner$$$outer();
    descriptor: ()LOuter;
    flags: (0x1011) ACC_PUBLIC, ACC_FINAL, ACC_SYNTHETIC