RFR: 8309203: C2: remove copy-by-value of GrowableArray for InterfaceSet

[Roland Westrelin]

The `InterfaceSet` component of types is immutable (once it is fully
constructed). In that way, it's similar to other type components. In
this change I propose making `InterfaceSet` a subtype of `Type` (and
renaming it `TypeInterfaces`). It's then feasible to `hashcons`
`TypeInterfaces` instances the way it's done for other `Type`
instance.  A single copy of a `TypeInterfaces` for a particular set of
interfaces is live during a compilation (and for the entire length of
the compilation) and pointer equality can be used to check two sets of
interfaces for equality. I think that change makes interface support
fit better with the type system implementation and has the added
benefits that there's no copy by value of the GrowableArray that holds
the set of interfaces anymore.

A couple of required virtual methods for `Type` are not implemented
for `TypeInterfaces`: `xmeet` and `singleton` (their body now calls
`ShouldNotReachHere`). They would need a way to tell whether
`TypeInterfaces` instances are above the center line or not. I thought
about (and tried) having a root class `Type` that would contain the
bare minimum for type that can be `hashcons`'ed so this method
wouldn't need to be implemented by `TypeInterfaces`. That proved to be
quite complicated and I gave up.

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Commit messages:
 - fix

Changes: https://git.openjdk.org/jdk/pull/16309/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=16309&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8309203
  Stats: 278 lines in 2 files changed: 66 ins; 44 del; 168 mod
  Patch: https://git.openjdk.org/jdk/pull/16309.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/16309/head:pull/16309

PR: https://git.openjdk.org/jdk/pull/16309