RFR: 8357053: ZGC: Improved utility for ZPageAge [v5]

[Joel Sikström]

> Hello,
> 
> This RFE improves utility for converting to/from, iterating over and defining structures that are indexed using the `ZPageAge` type.
> 
> Converting to/from ZPageAge and its underlying type (uint8_t, often just uint) is currently done via using static_cast. This works fine because sane values are converted in all use cases. However, to make conversion safer (and also more readable), I propose we add a `to_zpageage` and a corresponding `untype` that checks that the conversion is valid. Such conversion methods should be used instead of calling `static_cast<uint/ZPageAge>`.
> 
> We currently define a value called `ZPageAgeMax`, which is defined as `static_cast<uint>(ZPageAge::old)`. The majority of places that use this value actualy use `ZPageAgeMax + 1`, which is equivalent to the number of ages. Instead, I propose we define and use a value that represents the number of possible ages, called `ZPageAgeCount`.
> 
> Lastly, to make iterating over ages more accessible, I propose we create an intreface of enum iterators of ZPageAge. This will also create a foundation for generating values that require a ZPageAge in the future. Since the end of the enum iterators are exclusive, I've opted to use the following value as end for the iterators:
> 
> constexpr ZPageAge ZPageAgeLastPlusOne = static_cast<ZPageAge>(ZPageAgeCount);
> 
> 
> I see us using either this or a sentinel/dummy value at the end of the enum class, but I prefer having a value similar to `ZPageAgeLastPlusOne` over a dummy value.
> 
> Testing:
> * Oracle's tier 1-4
> * GHA

Joel Sikström has updated the pull request with a new target base due to a merge or a rebase. The pull request now contains 11 commits:

 - Merge branch 'master' into JDK-8357053_zpageage_utility
 - Remove redundant access specifier
 - Include order
 - Style fix :)
 - Added operator+/- for ZPageAge
 - Fix include order in enumIterator.hpp
 - Use T instead of EnumType
 - Use ENUMERATOR_RANGE instead of ENUMERATOR_VALUE_RANGE
 - Copyright years
 - Simplify untype(ZPageAge age)
 - ... and 1 more: https://git.openjdk.org/jdk/compare/07f5b762...3243a67a

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Changes: https://git.openjdk.org/jdk/pull/25251/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=25251&range=04
  Stats: 151 lines in 13 files changed: 93 ins; 9 del; 49 mod
  Patch: https://git.openjdk.org/jdk/pull/25251.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/25251/head:pull/25251

PR: https://git.openjdk.org/jdk/pull/25251