[clang] [clang] Add code completion for #embed directive in C23 mode (PR #165550)

[Nathan Ridge via cfe-commits]

HighCommander4 wrote:

> It looks like existing test coverage for preprocessor directive completions is found in [this file](https://searchfox.org/llvm/rev/3b30010303909efa01ac4d4fc018ad166e3b772d/clang/test/Index/complete-preprocessor.m). It's a bit of a weird test suite, but hopefully it should be clear from the existing test cases what the new one should look like; if not, I'm happy to go into more detail.

Note that you will need to either add `-std=c23` to an existing `RUN` line (e.g. see [this example](https://searchfox.org/llvm/rev/3b30010303909efa01ac4d4fc018ad166e3b772d/clang/test/Index/complete-lambdas.mm#25)), or add a new `RUN` line with `-std=c23`.

https://github.com/llvm/llvm-project/pull/165550