[clang-tools-extra] issue-63565: community requested small QoL fix for more configurabili… (PR #108005)

[Nathan Ridge via cfe-commits]

HighCommander4 wrote:

> Just to be clear, do you want to keep the new `--function-arg-placeholders` flags and also add it to the `.clangd` config!? Or do you only want the `.clangd` config and drop the changes to the `--function-arg-placeholders` flag!?

The latter. The only change related to the command-line flag that's needed, is to translate its (still boolean) value into the new (enum-valued) config option; we have a class named [FlagsConfigProvider](https://searchfox.org/llvm/rev/36ad0720de623221e3cc17d30f4173331c099a72/clang-tools-extra/clangd/tool/ClangdMain.cpp#638) which helps with that (so that the rest of the code only needs to care about the config option).

https://github.com/llvm/llvm-project/pull/108005