SPV: Tighten up number of struct-types declared based on decoration.

Takes some pressure off of issue #304.
Structures don't inherit locations and then explicitly decorate
members with them, so removed this reason to have another instance
of a structure type.

SPIRV/SpvBuilder.h

@@ -467,7 +467,7 @@ public:
 
     //
     // the SPIR-V builder maintains a single active chain that
-    // the following methods operated on
+    // the following methods operate on
     //
 
     // for external save and restore