[clangd] Also mark output arguments of array subscript expressions

... with the "usedAsMutableReference" semantic token modifier.
It's quite unusual to declare the index parameter of a subscript
operator as a non-const reference type, but arguably that makes it even
more helpful to be aware of it when working with such code.

Reviewed By: nridge

Differential Revision: https://reviews.llvm.org/D128892