c23 type compatibility rules, v2

[Martin Uecker <uecker@tugraz.at>]

This is a revised series for the C23 rules for type
compatibility.

1/6 c: reorganize recursive type checking
2/6 c23: recursive type checking of tagged type
3/6 c23: tag compatibility rules for struct and unions
4/6 c23: tag compatibility rules for enums
5/6 c23: aliasing of compatible tagged types
6/6 c23: construct composite type for tagged types
x/x c: flag for tag compatibility rules


1. simplifies type checking without functionality changes
as a preparation step. (This is based on a similar preparatory
patch I posted before for checking size expressions).

2. implements the new rules in comptypes for tagged types but 
the code still remains unused. This removes a lot of old
code because we now require union members to have the same
order and merges the code for structs and unions.

3. implements the rules for structs and unions.

4. does the same for enum types and enumerators. 

5. sets TYPE_CANONICAL based on a equivalence class of types
which makes aliasing work correctly. For this there is a new
comptypes_equiv_p that does relaxed checking (ignoring size
expressions in nested types but not for fields).

6. adds support for the composite type.

There is an extra patch that adds the a flag to activate
the compatibility rules independently from language mode
and activates it by default.

1-2 should cause no change in function. 3-6 implement the
new semantics for C23.

Bootstrapped and regression tested on x86_64 (also with the
extra patch).

Martin