Fix issue #163, support creation and reference of composite type
specialization constants.

e.g.:
```
layout(constant_id = 200) const float myfloat = 1.25;
layout(constant_id = 201) const int myint = 14;
struct structtype {
  float f;
  int i;
};
const structtype outer_struct_var = {myfloat, myint};
void main(){}
```
generated code (use glslangValidator):
```
// 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 Vertex 4  "main"
                              Source GLSL 450
                              Name 4  "main"
                              Name 10  "structtype"
                              MemberName 10(structtype) 0  "f"
                              MemberName 10(structtype) 1  "i"
                              Decorate 7 SpecId 200
                              Decorate 9 SpecId 201
               2:             TypeVoid
               3:             TypeFunction 2
               6:             TypeFloat 32
               7:    6(float) SpecConstant 1067450368
               8:             TypeInt 32 1
               9:      8(int) SpecConstant 14
  10(structtype):             TypeStruct 6(float) 8(int)
              11:10(structtype) SpecConstantComposite 7 9
         4(main):           2 Function None 3
               5:             Label
                              Return
                              FunctionEnd
```

Rname two function names to match their functionalities.
1) Rename `GlslangToSpvTraverser::createSpvSpecConstant()` to
`createSpvConstant()`;
2) Rename `GlslangToSpvTraverser::createSpvConstant()` to
`createSpvConstantFromConstUnionArray()`

Add function `GlslangToSpvTraverser::createSpvConstantFromSubTree()` to
handle constant creation from sub trees (e.g.: specialization constants).

Related PR: #208