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

glslang/CMakeLists.txt

@@ -132,6 +132,8 @@ target_include_directories(glslang PUBLIC
     $<BUILD_INTERFACE:${CMAKE_CURRENT_SOURCE_DIR}/..>
     $<INSTALL_INTERFACE:${CMAKE_INSTALL_INCLUDEDIR}>)
 
+glslang_only_export_explicit_symbols(glslang)
+
 if(WIN32 AND BUILD_SHARED_LIBS)
     set_target_properties(glslang PROPERTIES PREFIX "")
 endif()