WIP: track declared builtin type [proposal]

Marking as WIP since it might deserve discussion or at least explicit consideration.

During type sanitization, the TQualifier's TBuiltInVariable type is lost.  However,
sometimes it's needed downstream.  There were already two methods in use to track
it through to places it was needed: one in the TParameter, and one in a map in the
HlslParseContext used for structured buffers.

The latter was going to be insufficient when SB types with counters are passed to
user functions, and it's proving awkward to track the data to where it's needed.
This PR holds a proposal: track the original declared builtin type in the TType,
so it's trivially available where needed.

This lets the other two mechanisms be removed (and they are in this PR).  There's a
side benefit of not losing certain types of information before the reflection interface.

This PR is only that proposal, so it changes no test results.  If it's acceptable,
I'll use it for the last piece of SB counter functionality.

glslang/Include/Types.h

@@ -400,6 +400,7 @@ public:
         invariant = false;
         noContraction = false;
         makeTemporary();
+        declaredBuiltIn = EbvNone;
     }
 
     // drop qualifiers that don't belong in a temporary variable
@@ -457,6 +458,7 @@ public:
     const char*         semanticName;
     TStorageQualifier   storage   : 6;
     TBuiltInVariable    builtIn   : 8;
+    TBuiltInVariable    declaredBuiltIn : 8;
     TPrecisionQualifier precision : 3;
     bool invariant    : 1; // require canonical treatment for cross-shader invariance
     bool noContraction: 1; // prevent contraction and reassociation, e.g., for 'precise' keyword, and expressions it affects