[clang-doc] Fix failing tests on Windows

Tests on Windows were failing due to path separator differences.
'/' was being used as separator in the expected output, paths in expected
output are now changed to their native form before comparing them to the
actual output.

Committed on behalf of Diego Astiazarán (diegoaat97@gmail.com).

Differential Revision: https://reviews.llvm.org/D64669

llvm-svn: 365967