Update tasty format for lambda

[nicolasstucki]

No description provided.

[Blaisorblade]

The failure seems spurious (http://dotty-ci.epfl.ch/lampepfl/dotty/4365/4 has a timeout, while the same test on the parent succeeded http://dotty-ci.epfl.ch/lampepfl/dotty/4348/4), but

[error] Test dotty.tools.dotc.CompilationTests.runAll failed: java.lang.AssertionError: Run test failed, but should not, reasons:
[error]   - test 'tests/run-with-compiler/i3947f.scala' timed out, took 219.395 sec

is either misleading or worth investigating (why would a test take 3 minutes to run? was the machine that overloaded?)
EDIT: anyway, orthogonal to this change.

[nicolasstucki]

Yes, that test fails from time to time. I have another PR that should improve this. See #4242

[liufengyun]

Why there is no code change?

[nicolasstucki]

Because I am only updating the documentation to match the current implementation

[nicolasstucki]

See:
https://github.com/lampepfl/dotty/blob/master/compiler/src/dotty/tools/dotc/core/tasty/TreePickler.scala#L411-L417
and
https://github.com/lampepfl/dotty/blob/master/compiler/src/dotty/tools/dotc/core/tasty/TreeUnpickler.scala#L1022-L1025