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.)

Test/spv.unit1.frag

@@ -0,0 +1,14 @@
+#version 460
+
+float f;
+float a1;
+
+float foo();
+
+void main()
+{
+    f = 10;
+    float g = foo();
+    f += g;
+    f += gl_FragCoord.y;
+}