HLSL: Fix #1815: Don't constant fold spec-constant array indexing.

Test/hlsl.specConstant.frag

+[[vk::constant_id(0)]] const uint index = 2;
+
+static const uint array[] = { 10, 20, 30, 40 };
+
+float4 main( ) : SV_TARGET
+{
+    float4 r;
+    for(uint i = 0; i < array[index]; i++)
+        r = i;
+
+    r += index + index;
+    r += 2 * index;
+
+    return r;
+}

gtests/Hlsl.FromFile.cpp

@@ -351,6 +351,7 @@ INSTANTIATE_TEST_CASE_P(
         {"hlsl.shapeConvRet.frag", "main"},
         {"hlsl.self_cast.frag", "main"},
         {"hlsl.snorm.uav.comp", "main"},
+        {"hlsl.specConstant.frag", "main"},
         {"hlsl.staticMemberFunction.frag", "main"},
         {"hlsl.staticFuncInit.frag", "main"},
         {"hlsl.store.rwbyteaddressbuffer.type.comp", "main"},

hlsl/hlslParseHelper.cpp

@@ -816,7 +816,8 @@ TIntermTyped* HlslParseContext::handleBracketDereference(const TSourceLoc& loc,
                   base->getAsSymbolNode()->getName().c_str(), "");
         else
             error(loc, " left of '[' is not of type array, matrix, or vector ", "expression", "");
-    } else if (base->getType().getQualifier().storage == EvqConst && index->getQualifier().storage == EvqConst) {
+    } else if (base->getType().getQualifier().isFrontEndConstant() && 
+               index->getQualifier().isFrontEndConstant()) {
         // both base and index are front-end constants
         checkIndex(loc, base->getType(), indexValue);
         return intermediate.foldDereference(base, indexValue, loc);