Tweak clang-tools-extra/test/clang-tidy/file-filter.cpp to pass on win32.

FIXME: "-I %S/Inputs/file-filter/system/.." must be redundant.
On Win32, file-filter/system\system-header1.h precedes file-filter\header*.h due to code order between '/' and '\\'.

We should remove such a tweak to introduce the *right* path canonicalization.

Posix:
  file-filter/header*.h
  file-filter/system/system-header1.h

Win32:
  file-filter/system\system-header1.h
  file-filter\header*.h

Win32, tweaked:
  file-filter/system/..\header*.h
  file-filter/system\system-header1.h

It had been disabled since r220837.

llvm-svn: 245566