Go to Definition in #included TableGen files

I am not sure if this is a bug or not, or whether I'm using the tablegen language server incorrectly.

I modified the LLVM CMake to generate an entry in `tablegen_compile_commands.yml` for every single tablegen'd file. This contains duplicates (as the one on `main` does if you build and include MLIR in the enabled projects), but all duplicates have the same include list.

I'm working on the AArch64 backend (in `llvm-project/llvm/lib/Target/AArch64`), and I don't think that "Go To Definition" works in some cases.

For instance:
- In `AArch64.td`, trying "Go To Definition" on `SubtargetFeature<...` correctly takes me to the definition of `SubtargetFeature` in Target.td.
- In `AArch64InstrInfo.td`, trying "Go To Definition" on anything doesn't work (I also get a problem on the first definition of not being able to find the class). The same applies in e.g. `AArch64SystemOperands.td` There are not `filepath:` entries for these files, because they are not passed to llvm-tblgen directly.

Is this because the extension doesn't cope well with how LLVM normally structures the tablegen in backends? For instance, in the AArch64 backend, `AArch64.td` contains a few definitions, and then a lot of `#include` for the other `AArch64*.td` files, and it is only `AArch64.td` which is run through `llvm-tblgen`. Some of the definitions in the `AArch64*.td` files rely on definitions in `AArch64.td`, but do not `#include` that file because the inclusions go in the other direction.

Presumably there are methods to solve this, as C/C++ plugins do cope with definitions/declarations in header files (which do not normally show up in a compile_commands.json), but I don't know how complex that would be to implement.

Start by reproducing Go to Definition for SubtargetFeature in AArch64.td, then compare it with AArch64InstrInfo.td and AArch64SystemOperands.td. Inspect tablegen_compile_commands.yml and the llvm-tblgen entry points to understand how included files are represented. Done means definitions in included AArch64*.td files resolve without filepath entries.

Written by the indexing model from the issue text.