[clang] In DependencyCollector on Windows, ignore case and separators when...

[clang] In DependencyCollector on Windows, ignore case and separators when discarding duplicate dependency file paths.

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