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

[Sirui Mu via cfe-commits]

Lancern wrote:

> For the test, it would be better to write it in [this format](https://github.com/llvm/llvm-project/tree/main/clang/test/CodeCompletion) rather than using clangd.

Thanks for your helpful review. I'll move the test to [clang/test/CodeCompletion/overrides.cpp](https://github.com/llvm/llvm-project/blob/main/clang/test/CodeCompletion/overrides.cpp).

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