[clang][Tooling] Make the filename behaviour consistent

Dotdots were removed only when converting a relative path to absolute
one. This patch makes the behaviour consistent for absolute file paths by
removing them in that case as well. Also updates the documentation to mention
the behaviour.

Fixes https://github.com/clangd/clangd/issues/1317

Differential Revision: https://reviews.llvm.org/D137962