[clang][HeaderSuggestion] Handle the case of dotdot with an absolute path

Summary:
Include insertion in clangd was inserting absolute paths when the
include directory was an absolute path with a double dot. This patch makes sure
double dots are handled both with absolute and relative paths.

Reviewers: sammccall

Subscribers: ilya-biryukov, ioeric, jkorous, arphaman, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D60873

llvm-svn: 359078