HLSL: phase 3a: Add sub-vec4 rwtexture formats (qualifier.layoutFormat)

This PR sets the TQualifier layoutFormat according to the HLSL image type.
For instance:

  RWTexture1D <float2> g_tTex1df2;

becomes ElfRg32f.  Similar on Buffers, e.g, Buffer<float4> mybuffer;

The return type for image and buffer loads is now taken from the storage format.
Also, the qualifier for the return type is now (properly) a temp, not a global.

hlsl/hlslParseHelper.h

@@ -156,6 +156,8 @@ public:
     TIntermTyped* handleLvalue(const TSourceLoc&, const char* op, TIntermTyped* node);
     bool lValueErrorCheck(const TSourceLoc&, const char* op, TIntermTyped*) override;
 
+    TLayoutFormat getLayoutFromTxType(const TSourceLoc&, const TType&);
+
 protected:
     void inheritGlobalDefaults(TQualifier& dst) const;
     TVariable* makeInternalVariable(const char* name, const TType&) const;