[WIP] MDEV-35770 Augment DBUG_ASSERT with __builtin_assume

[UltraGreed]

• The Jira issue number for this PR is: MDEV-35770

Description

The DBUG_ASSERT macro tests an invariant at runtime in debug build and aborts if the check fails. Although assertions are disabled in optimized builds, code is usually written in such a manner, that failing an assertion condition would lead to undefined behavior. It therefore makes sense to introduce platform-specific analog of C++23 [[assume(cond)]] and use it in these cases, which potentially could lead to generation of more optimized builds in production.
E.g. compiler would be able to optimize while loop into do-while loop (which means less instructions in assembly) if it knows that the loop will run at least once.

However, there are lots of code fragments like:

DBUG_ASSERT(!thd->in_sub_stmt);
if (thd->in_sub_stmt)  {
  ...
}

and

switch (i)
{
  case 1: x=myisam_sint1korr(from); break;
  case 2: x=myisam_sint2korr(from); break;
  case 3: x=myisam_sint3korr(from); break;
  case 4: x=myisam_sint4korr(from); break;
  default: DBUG_ASSERT(0); x= 0;
}

If we simply wrap DBUG_ASSERT in an assume, the compiler will elide both the if-guard and the default branch — removing important fallback code.

This PR:

1. Introduces new platform-dependent macro DBUG_ASSUME, implemented for GCC >= 13.0, Clang and MSVC.
2. Augments DBUG_ASSERT to use it by default.
3. Provides DBUG_ASSERT_NO_ASSUME (identical to the old DBUG_ASSERT) for described above cases.

DBUG_ASSUME is implemented for GCC >= 13.0, Clang and MSVC.

TODO: Measure actual performance improvements and document LLVM/assembly diffs.

How can this PR be tested?

No additional test cases are needed, since this patch doesn't intend to change the behaviour. We may have missed a few “special” patterns, but we’ve covered all known occurrences.

Basing the PR against the correct MariaDB version

• This is a new feature or a refactoring, and the PR is based against the main branch.
• This is a bug fix, and the PR is based against the earliest maintained branch in which the bug can be reproduced.

PR quality check

• I checked the CODING_STANDARDS.md file and my PR conforms to this where appropriate.
• For any trivial modifications to the PR, I am ok with the reviewer making the changes themselves.

[CLAassistant]

All committers have signed the CLA.

[svoj]

Hi @UltraGreed! What is the intention of this pull request? Why is it submitted under MDEV-35770 umbrella? What kind of feedback are you expecting from MariaDB developers?

[svoj]

@FooBarrior is mentoring this effort.

[UltraGreed]

Hello, @svoj. Sorry for the delayed response. I've updated the PR to better reflect its intention. The only commit uploaded so far is a small fix (related to described in the PR problem), which will be squashed into the main commit later.