compiler.h: add const_true()

compiler.h: add const_true()

__builtin_constant_p() is known for not always being able to produce
constant expression [1] which led to the introduction of
__is_constexpr() [2]. Because of its dependency on
__builtin_constant_p(), statically_true() suffers from the same
issues.

For example:

  void foo(int a)
  {
  	 /* fail on GCC */
  	BUILD_BUG_ON_ZERO(statically_true(a));

  	 /* fail on both clang and GCC */
  	static char arr[statically_true(a) ? 1 : 2];
  }

For the same reasons why __is_constexpr() was created to cover
__builtin_constant_p() edge cases, __is_constexpr() can be used to
resolve statically_true() limitations.

Note that, somehow, GCC is not always able to fold this:

  __is_constexpr(x) && (x)

It is OK in BUILD_BUG_ON_ZERO() but not in array declarations nor in
static_assert():

  void bar(int a)
  {
  	/* success */
  	BUILD_BUG_ON_ZERO(__is_constexpr(a) && (a));

  	/* fail on GCC */
  	static char arr[__is_constexpr(a) && (a) ? 1 : 2];

  	/* fail on GCC */
  	static_assert(__is_constexpr(a) && (a));
  }

Encapsulating the expression in a __builtin_choose_expr() switch
resolves all these failed tests.

Define a new const_true() macro which, by making use of the
__builtin_choose_expr() and __is_constexpr(x) combo, always produces a
constant expression.

It should be noted that statically_true() is the only one able to fold
tautological expressions in which at least one on the operands is not a
constant expression. For example:

  statically_true(true || var)
  statically_true(var == var)
  statically_true(var * 0 + 1)
  statically_true(!(var * 8 % 4))

always evaluates to true, whereas all of these would be false under
const_true() if var is not a constant expression [3].

For this reason, usage of const_true() should be the exception.
Reflect in the documentation that const_true() is less powerful and
that statically_true() is the overall preferred solution.

[1] __builtin_constant_p cannot resolve to const when optimizing
Link: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=19449
[2] commit 3c8ba0d61d04 ("kernel.h: Retain constant expression output for max()/min()")
Link: https://git.kernel.org/torvalds/c/3c8ba0d61d04
[3] https://godbolt.org/z/c61PMxqbK

CC: Linus Torvalds <torvalds@linux-foundation.org>
CC: Rasmus Villemoes <linux@rasmusvillemoes.dk>
CC: Luc Van Oostenryck <luc.vanoostenryck@gmail.com>
Reviewed-by: Yury Norov <yury.norov@gmail.com>,
Signed-off-by: Vincent Mailhol <mailhol.vincent@wanadoo.fr>
Signed-off-by: Yury Norov <yury.norov@gmail.com>