Whole stack: Fix stale types in the AST linker object nodes, fixing #557.

Rationalizes the entire tracking of the linker object nodes, effecting
GLSL, HLSL, and SPIR-V, to allow tracked objects to be fully edited before
their type snapshot for linker objects.

Should only effect things when the rest of the AST contained no reference to
the symbol, because normal AST nodes were not stale. Also will only effect such
objects when their types were edited.

Test/baseResults/spv.noWorkgroup.comp.out

@@ -0,0 +1,34 @@
+spv.noWorkgroup.comp
+Warning, version 450 is not yet complete; most version-specific features are present, but some are missing.
+
+
+Linked compute stage:
+
+
+// Module Version 10000
+// Generated by (magic number): 80001
+// Id's are bound by 12
+
+                              Capability Shader
+               1:             ExtInstImport  "GLSL.std.450"
+                              MemoryModel Logical GLSL450
+                              EntryPoint GLCompute 4  "main"
+                              ExecutionMode 4 LocalSize 1 1 1
+                              Source GLSL 450
+                              Name 4  "main"
+                              Decorate 7 SpecId 18
+                              Decorate 8 SpecId 10
+                              Decorate 9 SpecId 19
+                              Decorate 11 BuiltIn WorkgroupSize
+               2:             TypeVoid
+               3:             TypeFunction 2
+               6:             TypeInt 32 0
+               7:      6(int) SpecConstant 1
+               8:      6(int) SpecConstant 1
+               9:      6(int) SpecConstant 1
+              10:             TypeVector 6(int) 3
+              11:   10(ivec3) SpecConstantComposite 7 8 9
+         4(main):           2 Function None 3
+               5:             Label
+                              Return
+                              FunctionEnd