C++: Support more complex 16-bit float types

Author: jketema

cpp/downgrades/7bc12b02a4363149f0727a4bce07952dbb9d98aa/builtintypes.ql

@@ -0,0 +1,14 @@
+class BuiltinType extends @builtintype {
+  string toString() { none() }
+}
+
+from BuiltinType type, string name, int kind, int kind_new, int size, int sign, int alignment
+where
+  builtintypes(type, name, kind, size, sign, alignment) and
+  if
+    type instanceof @complex_fp16 or
+    type instanceof @complex_std_bfloat16 or
+    type instanceof @complex_std_float16
+  then kind_new = 2
+  else kind_new = kind
+select type, name, kind_new, size, sign, alignment