Properly detect item under cursor when within a macro

[SkidanovAlex]

For the line

assert(moo == 5);

if my cursor is on the moo, the actual symbol under cursor is assert macro, because Clang doesn't know that assert's argument is just inserted into code without any modifications. Same is for CHECK and Auto, so neither looking for definitions nor looking for references work -- definition/references of assert/CHECK/Auto are shown instead.