Add --no-link option

Adds the --no-link option which outputs the compiled shader binaries
without linking them. This is a first step towards allowing users to
create SPIR-v binary, non-executable libraries.

When using the --no-link option, all functions are decorated with the
Export linkage attribute.

Test/baseResults/spv.exportFunctions.comp.out

@@ -0,0 +1,36 @@
+spv.exportFunctions.comp
+// Module Version 10000
+// Generated by (magic number): 8000b
+// Id's are bound by 22
+
+                              Capability Shader
+                              Capability Linkage
+               1:             ExtInstImport  "GLSL.std.450"
+                              MemoryModel Logical GLSL450
+                              Source GLSL 450
+                              Name 7  "add(f1;f1;"
+                              Name 5  "a"
+                              Name 6  "b"
+                              Name 11  "foo("
+                              Decorate 7(add(f1;f1;) Linkage Attributes 6579297 0
+                              Decorate 11(foo() Linkage Attributes 7303014 0
+               2:             TypeFloat 32
+               3:             TypePointer Function 2(float)
+               4:             TypeFunction 2(float) 3(ptr) 3(ptr)
+               9:             TypeInt 32 1
+              10:             TypeFunction 9(int)
+              17:             TypeVoid
+              19:      9(int) Constant 0
+   7(add(f1;f1;):    2(float) Function None 4
+            5(a):      3(ptr) FunctionParameter
+            6(b):      3(ptr) FunctionParameter
+               8:             Label
+              13:    2(float) Load 5(a)
+              14:    2(float) Load 6(b)
+              15:    2(float) FAdd 13 14
+                              ReturnValue 15
+                              FunctionEnd
+        11(foo():      9(int) Function None 10
+              12:             Label
+                              ReturnValue 19
+                              FunctionEnd

Test/spv.exportFunctions.comp

@@ -0,0 +1,9 @@
+#version 450
+
+float add(float a, float b) {
+    return a + b;
+}
+
+int foo() {
+    return 0;
+}