[clang-tools-extra] [clangd] Add inlay hints for default function arguments and implicit lambda captures (PR #95712)

[Tor Shepherd via cfe-commits]

torshepherd wrote:

Ok I took a stab at doing lambda captures as hover instead.

- Targeting the exact '=' or '&' is nontrivial since the capture list isn't in the AST, the lambda is just `LambdaExpr` with children for the parameters and the CompoundStmt body.
- `auto MyLambda = [...` already produces a nice function-like hover over the variable. I tested adding a "Captures:" bullet-list, it works well enough but isn't very noticeable.
- We could take the above HoverInfo over the variable containing the lambda and apply it to hovering the lambda itself as well?


It strikes me, if we could either add HoverInfo or styling to the variables within the body of the lambda themselves, that would be the most readable representation probably.

For instance:

- Can we add an inlay hint like `[&] ` before actual variables in the body of the lambda?
- Can we apply a semantic token to style lambda captures differently? Maybe as `property` since they are similar conceptually to struct members?
- Can we use the `mutable`-ness of the lambda to color tokens as const if they are captured by copy and the lambda is not mutable?

Overall I think capture hints should be removed from this PR so we can get default args in in the meantime.

https://github.com/llvm/llvm-project/pull/95712