[fix][clang-tidy] fix for r345961 that introduced a test failure on Windows builds

r345961 introduced a path check in
.\tools\clang\tools\extra\test\clang-tidy\clang-tidy-run-with-database.cpp.

r345961 added a check line for a path that only handled / on unix
machines and not \ on windows machines.

This patch handles both cases.

Patch by TWeaver.

Differential Revision: https://reviews.llvm.org/D54036

llvm-svn: 345995