[clang] [clang-tools-extra] [Sema][clangd] add noexcept to override functions during code completion (PR #75937)

[Sirui Mu via cfe-commits]

Lancern wrote:

> The place you're patching is not only specific to "completing override functions", but handles all completion strings involving function declarations.

OK. I'll move the changes to the `CodeCompletionResult::createCodeCompletionStringForOverride` function which seems like a more appropriate place for this patch to me.

> Bonus: It appears that neither gcc nor clang implements a provision change from [CWG1351](https://cplusplus.github.io/CWG/issues/1351.html),

Maybe this deserves a new issue for clang Sema?

https://github.com/llvm/llvm-project/pull/75937