[clang] [Clang] Deleting an incomplete enum type is not an error (PR #118455)

[Alexander Kornienko via cfe-commits]

alexfh wrote:

To avoid this I usually incorporate a validity check in my interestingness test, e.g. by running a known-good clang on the candidate input, e.g. currently I use:
```
$ cat bad-clang-crashes.sh
#!/bin/bash -eux
ARGS="-fdiagnostics-show-option -Wall -Werror -Wno-inconsistent-missing-override -Wno-unused -Wno-unused-lambda-capture -Wno-deprecated -Wno-mismatched-tags -Wno-gnu-alignof-expression -Wno-deprecated-this-capture -std=gnu++20 -Wno-defaulted-function-deleted -fsyntax-only -Wno-user-defined-literals -Wno-nontrivial-memaccess -Wno-invalid-constexpr -Wno-misleading-indentation -Wno-dynamic-class-memaccess -Wno-ignored-attributes"

$(dirname $0)/clang-good $ARGS q.cc &
pid=$!
$(dirname $0)/clang-bad-checked $ARGS q.cc 2>&1 | grep "Declaration context must already be complete"
wait $pid
```

https://github.com/llvm/llvm-project/pull/118455