Fix unsafe globals in Interval and Region3

[hidmic]

🦟 Bug fix

Relates to #269.

Summary

This PR fixes a potential global initialization order issue introduced in #388 and #390 by making all Interval and Region3 construction forms constexpr.

Checklist

• Signed all commits for DCO
• Added tests
• Updated documentation (as needed)
• Updated migration guide (as needed)
• codecheck passed (See contributing)
• All tests passed (See test coverage)

Note to maintainers: Remember to use Squash-Merge and edit the commit message to match the pull request summary while retaining Signed-off-by messages.

[codecov]

Codecov Report

Merging #396 (e9095d1) into main (dcde2c0) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main     #396   +/-   ##
=======================================
  Coverage   99.73%   99.73%           
=======================================
  Files          69       69           
  Lines        6353     6353           
=======================================
  Hits         6336     6336           
  Misses         17       17           

Impacted Files	Coverage Δ
include/ignition/math/Interval.hh	100.00% <100.00%> (ø)
include/ignition/math/Region3.hh	100.00% <100.00%> (ø)

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[chapulina]

LGTM, but I'd like to make sure @scpeters approves too before merging

[scpeters]

it seems right to me, but I think @azeey understands this issue better than me. I think the right way to test this is with asan using gazebosim/gz-cmake#210, but I haven't had a chance to do so. I think it's fine to merge without my approval. Once we get CI with sanitizers, we'll know for sure :)

[chapulina]

Sounds reasonable, we can revisit when the sanitizers are running in CI, if needed