RFR: 8337563: NMT: rename MEMFLAGS to MemTag [v11]

[Gerard Ziemski]

> Please review this cleanup, where we rename `MEMFLAGS` to `MemTag`.
> 
> `MEMFLAGS` implies that we can use more than one at the same time, but those are exclusive values, so `MemTag` is a more suitable name.
> 
> This fix also includes a cleanup of all the related function/template parameter names and local variable names.
> 
> Testing is pending...
> 
> Note: there is more history in old closed PRs [https://github.com/openjdk/jdk/pull/20497](https://github.com/openjdk/jdk/pull/20497) and [https://github.com/openjdk/jdk/pull/20472](https://github.com/openjdk/jdk/pull/20472)

Gerard Ziemski has updated the pull request incrementally with two additional commits since the last revision:

 - copyrights
 - Afshin's feedback, tests

-------------

Changes:
  - all: https://git.openjdk.org/jdk/pull/20872/files
  - new: https://git.openjdk.org/jdk/pull/20872/files/1765579b..7db989f3

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=20872&range=10
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=20872&range=09-10

  Stats: 9 lines in 7 files changed: 1 ins; 0 del; 8 mod
  Patch: https://git.openjdk.org/jdk/pull/20872.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/20872/head:pull/20872

PR: https://git.openjdk.org/jdk/pull/20872