Encapsulate Loc member access

[dkorpel]

Using functions instead of direct field access allows Loc to change its internals more easily, which helps with the recent effort to reduce its size: struct Location size

[dlang-bot]

Thanks for your pull request and interest in making D better, @dkorpel! We are looking forward to reviewing it, and you should be hearing from a maintainer soon.
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Testing this PR locally

If you don't have a local development environment setup, you can use Digger to test this PR:

dub run digger -- build "master + dmd#15211"

[ibuclaw]

Loc could be replaced with a uint (index into an array, or hash table), then it's internals (and size) will be completely hidden.

[dkorpel]

Loc could be replaced with a uint (index into an array, or hash table)

Yes, that's the plan