HLSL: Correct some mistakes for min16 types

- Add missing constructor ops to support float16/int16/uint16 types
- Allow half float literals
- Correct two errors of double literal parse in HLSL: extension check and
  postfix

hlsl/hlslGrammar.cpp

@@ -3283,6 +3283,9 @@ bool HlslGrammar::acceptLiteral(TIntermTyped*& node)
     case EHTokUintConstant:
         node = intermediate.addConstantUnion(token.u, token.loc, true);
         break;
+    case EHTokFloat16Constant:
+        node = intermediate.addConstantUnion(token.d, EbtFloat16, token.loc, true);
+        break;
     case EHTokFloatConstant:
         node = intermediate.addConstantUnion(token.d, EbtFloat, token.loc, true);
         break;