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.unit3.frag

@@ -0,0 +1,11 @@
+#version 460
+
+float f;
+float h3 = 3.0;
+
+float bar()
+{
+    h3 *= f;
+    float g3 = 2 * h3;
+    return h3 + g3 + gl_FragCoord.y;
+}