This change adds GLSLANG_EXPORT in a couple more places, as well as
adding a new symbol visibility annotation GLSLANG_EXPORT_FOR_TESTS which
is defined the same as GLSLANG_EXPORT but documents the intention that
the symbols marked with it are only meant to be used by glslang's test
suite and do not form part of the public API and are thus not subject to
ABI stability guarantees.
Symbols in the SPIRV/ directory that are meant to be part of the public
API of glslangnow have GLSLANG_EXPORT annotations that make sure they
will be accessible when building glslang as a shared library if
-fvisibility=hidden is enabled.
