glslang: Only export public interface for SOs

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

StandAlone/StandAlone.cpp

@@ -63,7 +63,7 @@
 #include "../glslang/OSDependent/osinclude.h"
 
 extern "C" {
-    SH_IMPORT_EXPORT void ShOutputHtml();
+    GLSLANG_EXPORT void ShOutputHtml();
 }
 
 // Command-line options