Add --no-link option

Adds the --no-link option which outputs the compiled shader binaries
without linking them. This is a first step towards allowing users to
create SPIR-v binary, non-executable libraries.

When using the --no-link option, all functions are decorated with the
Export linkage attribute.

glslang/MachineIndependent/ParseHelper.h

@@ -196,6 +196,7 @@ public:
     struct TPragma contextPragma;
     int beginInvocationInterlockCount;
     int endInvocationInterlockCount;
+    bool compileOnly = false;
 
 protected:
     TParseContextBase(TParseContextBase&);