Default to `-fvisibility=hidden`, and annotate the public glslang interface with `GLSLANG_EXPORT` to change the visibility of these cherry-picked symbols to default.
This is also used by Windows builds for `__declspec(dllexport)`-ing the public DLL interface.
This allows us to classify API changes into those that are publicly backwards compatible, and those that are not.
Note that `libSPIRV` will likely need similar treatment.
Issues: #2283, #1484
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