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.)

gtests/Link.FromFile.Vk.cpp

@@ -106,6 +106,7 @@ INSTANTIATE_TEST_CASE_P(
     Glsl, LinkTestVulkan,
     ::testing::ValuesIn(std::vector<std::vector<std::string>>({
         {"link1.vk.frag", "link2.vk.frag"},
+        {"spv.unit1.frag", "spv.unit2.frag", "spv.unit3.frag"},
     })),
 );
 // clang-format on