C++: Add support for C23/C++23 floating-point types

Author: jketema

cpp/downgrades/5b388693c66db1e7dc2e76a90aa67a2b6eb74f0f/builtintypes.ql

@@ -0,0 +1,19 @@
+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 @fp16 or
+    type instanceof @std_bfloat16 or
+    type instanceof @std_float16 or
+    type instanceof @complex_std_float32 or
+    type instanceof @complex_float32x or
+    type instanceof @complex_std_float64 or
+    type instanceof @complex_float64x or
+    type instanceof @complex_std_float128
+  then kind_new = 2
+  else kind_new = kind
+select type, name, kind_new, size, sign, alignment