[clang][HeaderSuggestion] Handle the case of dotdot with an absolute path
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.
Subscribers: ilya-biryukov, ioeric, jkorous, arphaman, cfe-commits
Differential Revision: https://reviews.llvm.org/D60873