Remove use of the register keyword

[dgovil]

Description of Change(s)

The register keyword is no longer allowed as of C++17. It's also now considered to be a fairly weak hint that doesn't do anything in most scenarios.
As a result, I've removed all uses of it in the lexer files, which allows USD to build as a C++17 project. The builds seem to work just fine without it, and some brief testing shows no difference in performance.

• I have verified that all unit tests pass with the proposed changes

• I have submitted a signed Contributor License Agreement (Under the Apple CLA)

[spiffmon]

Thanks, @dgovil - we're looking into upgrading our truly ancient version of flex to one that has the fix. If we don't manage to pull that off for our VFX2022 toolset, we'll definitely take this PR as a stop-gap.

[sunyab]

Filed as internal issue #USD-7923

[stilllman]

We had to make the same fix for C++17 compatibility, I was about to push a PR when I saw this one. Note that the register keyword is used in two other files, pxr/base/arch/stackTrace.cpp and third_party/renderman-24/plugin/hdPrman/virtualStructConditionalGrammar.lex.cpp.

[dgovil]

Ah good call out. I'll remove it there too and update this PR just in case it's useful.

@spiffmon were there any updates on changing your flex version? Would it make sense to accept this PR in the interim, since they wouldn't conflict?

[dgovil]

@stilllman updated the PR with the files you suggested. A quick ripgrep search didn't show other uses, so thanks for catching those.