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
Sylvain Audi