[clang] [clang] more useful error message for decomposition declaration missing initializer (PR #127924)

[Mariya Podchishchaeva via cfe-commits]

https://github.com/Fznamznon commented:

Thank you for the patch! The change needs a LIT test though. An example of a LIT test can be found in clang/test/SemaCXX. You would need to use -verify flag and check that the new error message is emitted as intended.  It would be great also to include a release note to clang/doc/ReleaseNotes.md explaining the improvement.
Please also note that precommit check identified a failure in one of clang-tidy tests (see https://buildkite.com/llvm-project/github-pull-requests/builds/148753#019520e6-9618-4cea-9524-69dbfadd249c ) due to wording of the diagnostic message changed.

https://github.com/llvm/llvm-project/pull/127924