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.unit2.frag

@@ -0,0 +1,14 @@
+#version 410
+// a different version number makes different id's for the same shared symbol
+
+float a2;
+float f;
+
+float bar();
+
+float foo()
+{
+    float h2 = 2 * f;
+    float g2 = bar();
+    return h2 + g2 + gl_FragCoord.y;
+}