Link/SPV: Correct symbol IDs on merging ASTs to a single coherent space

This is one step in providing full linker functionality for creating
correct SPIR-V from multiple compilation units for the same stage.
(This was the only remaining "hard" part. The rest should be simple.)

glslang/Include/intermediate.h

@@ -1164,6 +1164,7 @@ public:
         constSubtree(nullptr)
           { name = n; }
     virtual int getId() const { return id; }
+    virtual void changeId(int i) { id = i; }
     virtual const TString& getName() const { return name; }
     virtual void traverse(TIntermTraverser*);
     virtual       TIntermSymbol* getAsSymbolNode()       { return this; }