Add @hidden UDA

[kinke]

To hide a symbol in the sense of making it DSO-local. An export visibility is stronger and overrides the UDA.

On non-Windows and non-Mac, also newly hide symbols (without export) with linkonce_odr etc. linkage. The Mac linker warns about hidden linkonce_odr symbols, e.g.:

ld: warning: direct access in function '__D3std6format5write__T14formattedWriteTSQBm5stdio4File17LockingTextWriterTaTSQCx9container5array__T5ArrayTiZQjZQDlFKQCzMxAaQBvZk' from file '{{RESULTS_DIR}}/runnable/ldc_github_1033_0.o' to global weak symbol '__D44TypeInfo_S3std5array__T8AppenderTAyaZQo4Data6__initZ' from file '/Users/runner/work/1/build/lib/libphobos2-ldc.a(array.o)' means the weak symbol cannot be overridden at runtime. This was likely caused by different translation units being compiled with different visibility settings.

[kinke]

Ah wait, the Apple linker probably warns about a conflict between a public weak_odr TypeInfo symbol in regularly-compiled Phobos and a newly hidden linkonce_odr duplicate in an object file compiled with -linkonce-templates. That would make some sense to me. (dmd-testsuite debug was green, dmd-testsuite release with -linkonce-templates had failures.)

[JohanEngelen]

It'll be useful for other use cases anywhere, nice.

[kinke]

Yeah, more power for power users. :)

Wrt. export overriding the UDA, that's motivated by the visibility being downgradable via public or whatever, whereas you cannot remove the UDA from a single declaration in an @hidden scope.