Final round of symbol visibility fixes

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.

glslang/MachineIndependent/localintermediate.h

@@ -1063,6 +1063,7 @@ public:
     int checkLocationRT(int set, int location);
     int addUsedOffsets(int binding, int offset, int numOffsets);
     bool addUsedConstantId(int id);
+    GLSLANG_EXPORT_FOR_TESTS
     static int computeTypeLocationSize(const TType&, EShLanguage);
     static int computeTypeUniformLocationSize(const TType&);