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

This reverts commit 1dc0a1e5.

Failures were caused by unintentional conversion to native slashes by
remove_dots, so undo that: we always suggest posix slashes for includes.

This could potentially be a change in behavior on windows if people were
spelling headers with backslashes and headermaps contained backslashes,
but that's all underspecified and I don't think anyone uses headermaps
on windows.

Differential Revision: https://reviews.llvm.org/D138709