[llvm] Improve diagnostic for malformed constant vectors (PR #216253)

[via llvm-commits]

github-actions[bot] wrote:

<!--PREMERGE ADVISOR COMMENT: Windows-->
# :window: Windows x64 Test Results

* 135531 tests passed
* 3728 tests skipped
* 4697 tests failed

Failed tests and their output was too large to report. Download the build's log file to see the details.

If these failures are unrelated to your changes (for example tests are broken or flaky at HEAD), please open an issue at https://github.com/llvm/llvm-project/issues and add the `infrastructure` label.

https://github.com/llvm/llvm-project/pull/216253