[Lex] Fix suggested spelling of /usr/bin/../include/foo

Since D60873 we remove dotdots from the search path entries, but not the
filenames we're matching against, so do the latter too.

Since this also removes (single) dots, drop the logic to skip over them.
(Some of this was already dead, some is newly dead).

See D138676 for motivation.

Differential Revision: https://reviews.llvm.org/D138677