[clang] [clang-tools-extra] [clangd] Show struct fields and enum members in hovers (PR #89557)

[Nathan Ridge via cfe-commits]

HighCommander4 wrote:

I'm planning to revise the patch to make the following changes:

 1. Put the new behaviour behind a config option (I'm thinking `Hover` --> `ShowFields`)
 2. Add C language mode tests
 3. Use `PrintingCallbacks` instead of a `PrintingPolicy` flag

https://github.com/llvm/llvm-project/pull/89557