Fix use of decl in its own init

e.g.
x = (__typeof(x))...;
Show constant folding option
type_max() takes unsigned into account
const_op_exec() handles unsigned types properly
unsigned -> signed test
Moved -ftrapv test
Integer overflow/underflow const op test
Change const_op_exec() flow control - single return
Generalise overflow handling in constant-op code
Change when we fold a type - need __attribute__s
Add locations to types - may be skipped on creation
Only add to scope if we have a scope - fixes null pointer
If a decl doesn't have a symbol, it's not ready
Remove extra warning in typedef/init/func test
Merge branch 'fix/typeof-in-init'

Conflicts:
	src/cc1/fold.h
	src/cc1/parse_type.c
Fix phi merging with different typed operands

Short-circuit expressions were failing to compile correctly due to
the smaller operand having unclear/unzero higher bits in its version
of the merge register.
unsigned overflow test
Backend respects -fno-const-fold
Make -ftrapv apply to unary operators
Fix x86_64 idiv with unsigned types