[clang-tools-extra] [clangd] Store full decl/def range with symbol locations (PR #118102)

[Christian Kandeler via cfe-commits]

ckandeler wrote:

> 2. A more general musing: it would be nice to have a mechanism for storing additional information for a subset of symbols, without incurring the overhead of increasing the representation size of **every** symbol. If we had such a mechanism, we could cut down the overhead of this patch further, by e.g. only storing the decl/def range for functions rather than all symbol types.
>    
>    * Perhaps this could take the form of a pointer field in `Symbol` which for a subset of symbols points to additional data stored in the symbol slab? That would, in and of itself, not reduce the memory footprint compared to (1), since the pointer would take up 8 bytes, the same as a decl/def range stored inline. However, we've had use cases like this come up before. (An example that comes to mind is storing all definitions for symbols with multiple definitions; see [this comment](https://github.com/clangd/clangd/issues/1673#issuecomment-1594211998) and the next few.) So maybe it would be forward-looking to pay the cost for that pointer once, and unlock the ability to store various extra data in the future?

I notice that there are four fields in the Symbol struct that are only relevant for symbols indexed for completion. Presumably these would be candidates for such an extension block?



https://github.com/llvm/llvm-project/pull/118102