clang: Disable building dexp in clang-native

[ycongal-smile]

Building dexp (native) on Debian 11 causes intermittent failure0. This tools in not used in clang except in tests (that we don't run).

So, disable building this tool by encapsulating its build in a cmake option and then in a PACKAGECONFIG.

Users sure of not using Debian 11 (or the impacted g++ compiler) can turn this config on safely.

Only the -native* variants change. Dexp is still enabled by default for target.

---

Contributor checklist

• Changes have been tested
  • Built clang-native
  • Verified that Dexp.cpp has not been compiled
• Signed-off-by is present
• The PR complies with the Open Embedded Commit Patch Message Guidelines

Reviewer Guidelines

• When submitting a review, please pick:
  • 'Approve' if this change would be acceptable in the codebase (even if there are minor or cosmetic tweaks that could be improved).
  • 'Request Changes' if this change would not be acceptable in our codebase (e.g. bugs, changes that will make development harder in future, security/performance issues, etc).
  • 'Comment' if you don't feel you have enough information to decide either way (e.g. if you have major questions, or you don't understand the context of the change sufficiently to fully review yourself, but want to make a comment)

[ycongal-smile]

v2:

• rename the PACKAGECONFIG to clang-dexp to make the dependency to clangd clear.
• Don't try to enable it on target variant since clangd is disabled.

[ycongal-smile]

V3: update patch status and message to match the submitted PR: llvm/llvm-project#133124

[ycongal-smile]

v4: updated patch following results of the LLVM/CLANG CI. (Error only impacted the upstream tests)