[clangd] Add "usedAsMutablePointer" highlighting modifier

Counterpart to "usedAsMutableReference". Just as for references, there
are const and non-const pointer parameters, and it's valuable to be able
to have different highlighting for the two cases at the call site.
We could have re-used the existing modifier, but having a dedicated one
maximizes client flexibility.

Reviewed By: nridge

Differential Revision: https://reviews.llvm.org/D130015