HLSL: add helper access methods to TAttributeMap

There was some code replication around getting string and integer
values out of an attribute map.  This adds new methods to the
TAttributeMap class to encapsulate some accessor details.

hlsl/hlslAttributes.cpp

@@ -36,6 +36,7 @@
 #include "hlslAttributes.h"
 #include <cstdlib>
 #include <cctype>
+#include <algorithm>
 
 namespace glslang {
     // Map the given string to an attribute enum from TAttributeType,
@@ -131,4 +132,51 @@ namespace glslang {
         return attributes.find(attr) != attributes.end();
     }
 
+    // extract integers out of attribute arguments stored in attribute aggregate
+    bool TAttributeMap::getInt(TAttributeType attr, int& value, int argNum) const 
+    {
+        const TConstUnion* intConst = getConstUnion(attr, EbtInt, argNum);
+
+        if (intConst == nullptr)
+            return false;
+
+        value = intConst->getIConst();
+        return true;
+    };
+
+    // extract strings out of attribute arguments stored in attribute aggregate.
+    // convert to lower case if converToLower is true (for case-insensitive compare convenience)
+    bool TAttributeMap::getString(TAttributeType attr, TString& value, int argNum, bool convertToLower) const 
+    {
+        const TConstUnion* stringConst = getConstUnion(attr, EbtString, argNum);
+
+        if (stringConst == nullptr)
+            return false;
+
+        value = *stringConst->getSConst();
+
+        // Convenience.
+        if (convertToLower)
+            std::transform(value.begin(), value.end(), value.begin(), ::tolower);
+
+        return true;
+    };
+
+    // Helper to get attribute const union.  Returns nullptr on failure.
+    const TConstUnion* TAttributeMap::getConstUnion(TAttributeType attr, TBasicType basicType, int argNum) const
+    {
+        const TIntermAggregate* attrAgg = (*this)[attr];
+        if (attrAgg == nullptr)
+            return nullptr;
+
+        if (argNum >= int(attrAgg->getSequence().size()))
+            return nullptr;
+
+        const TConstUnion* constVal = &attrAgg->getSequence()[argNum]->getAsConstantUnion()->getConstArray()[0];
+        if (constVal == nullptr || constVal->getType() != basicType)
+            return nullptr;
+        
+        return constVal;
+    }
+
 } // end namespace glslang