fix #32785, implement typedef equality in dartdevc

This change expands typedefs into their underlying function type in the
compiler. Typedefs no longer exist at runtime in DDC, so they now have
similar equality and identity behavior as other function types.

The compiler used to preserve typedefs so they could have a better
toString. But over time that support has been almost entirely lost;
the caching for function/interface types eliminates typedefs, and
DDC+Kernel does not appear to get any TypedefTypes from the common
front end. So in practice typedef types were almost never present at
runtime. Because of this, we can remove the remaining support with very
little user visible effect. This also brings DDC's canonicalization
roughly in line with dart2js for compile-time typedefs, so frameworks
like Angular will be able to continue their use of `identical` for fast
type comparisons.

This CL also fixes DDC's type caching for generic function types
(typeHashCode was incorrect for them), and has some refactoring to
improve names/comments and make the SDK runtime code more clear.

Change-Id: I4e34c0c8f737a8535ba474a4683967ff41bcd3c8
Reviewed-on: https://dart-review.googlesource.com/74664
Commit-Queue: Jenny Messerly <jmesserly@google.com>
Reviewed-by: Alan Knight <alanknight@google.com>