[clangd] Add ObjC method support to prepareCallHierarchy

This fixes "textDocument/prepareCallHierarchy" in clangd for ObjC methods. Details at https://github.com/clangd/vscode-clangd/issues/247.

clangd uses Decl::isFunctionOrFunctionTemplate to check if the decl given in a prepareCallHierarchy request is eligible for prepareCallHierarchy. We change to use isFunctionOrMethod which includes functions and ObjC methods.

Reviewed By: kadircet

Differential Revision: https://reviews.llvm.org/D114058