[DebugInfo] Remove dots from getFilenameByIndex return value

When concatenating directory with filename in getFilenameByIndex, we
might end up with a path that contains extra dots. For example, if the
input is /path and ./example, we would return /path/./example. Run
sys::path::remove_dots on the output to eliminate unnecessary dots.

Differential Revision: https://reviews.llvm.org/D87657