This fixes#117438.
If paths in dependency file are not absoulte, make (or ninja) will
canonicalize them.
While their canonicalization does not involves symbolic links expansion
(for IO performance concerns), leaving a non-absolute path in dependency
file may lead to unexpected canonicalization.
For example, '/a/../b', where '/a' is a symlink to '/c/d', it should be
'/c/b' but make (and ninja) canonicalizes it as '/b', and fails for file
not found.
This patch removes duplicates also encountered in the output of clang-scan-deps when one same header file is encountered with different casing and/or different separators ('/' vs '\').
The case of separators can appear when the same file is included externally by
`#include <folder/file.h>`
whereas a file from the same folder does
`#include "file.h"`
Under Windows, clang computes the paths using '/' from the include directive, the `\` from the -I options, and the concatenations use the native `\`, leading to internal paths containing a mix of both separators.
Differential Revision: https://reviews.llvm.org/D102339
