[clang-tools-extra] [clangd] Support go-to-definition on type hints. The core part (PR #86629)

[Younan Zhang via cfe-commits]

zyn0217 wrote:

@HighCommander4 Given that this PR has been growing larger, I plan to split it up. Hopefully, this will make the code review easier.

Moreover, I realized it's probably insufficient to build the whole thing on top of a `TypeVisitor`. Like, a pointer to an array, we have to handle it in two parts so that we are able to put the asterisk in the middle of the array rather than at the end of it. (e.g. `int (*)[4]` vs `int [4] *`, where the latter is wrong.) So, we probably have to implement a mechanism similar to `TypePrinter`.

https://github.com/llvm/llvm-project/pull/86629