Vulkan: Finish semantics for what creates spec-const-semantics.

Note: This required adding a new test mode to see the AST for vulkan tests.
This also required reworking some deeper parts of type creation, regarding
when storage qualification and constness is deduced bottom-up or dictated
top-down.

Test/vulkan.ast.vert

@@ -0,0 +1,42 @@
+#version 450
+
+layout(constant_id = 200) const float scf1 = 1.0;
+layout(constant_id = 201) const bool scbt = true;
+layout(constant_id = 202) const int sci2 = 2;
+
+void main()
+{
+    bool(scf1);   // not a spec-const
+    bool(scbt);   // spec-const
+    bool(sci2);   // spec-const
+
+    float(scf1);   // not a spec-const
+    float(scbt);   // not a spec-const
+    float(sci2);   // not a spec-const
+
+    int(scf1);   // not a spec-const
+    int(scbt);   // spec-const
+    int(sci2);   // spec-const
+
+    scf1 * scf1;   // not a spec-const
+    scbt || scbt;  // spec-const
+    sci2 * sci2;   // spec-const
+    scf1 + sci2;   // implicit conversion not a spec-const
+
+    -scf1;     // not a spec-const
+    !scbt;     // spec-const
+    -sci2;     // spec-const
+
+    scf1 > scf1;   // not a spec-const
+    sci2 > sci2;   // spec-const
+
+    scf1 != scf1;  // not a spec-const
+    scbt != scbt;  // spec-const
+    sci2 != sci2;  // spec-const
+
+    ivec2(sci2, sci2);   // spec-const
+    ivec2[2](ivec2(sci2, sci2), ivec2(sci2, sci2)); // not a spec-const
+
+    vec2(scf1, scf1);   // not spec-const
+    vec2[2](vec2(scf1, scf1), vec2(scf1, scf1)); // not a spec-const
+}