This change adds support for cross-file diagnostic paths in html output. If the
diagnostic path is not cross-file, there is no change in the output.

Patch by Vlad Tsyrklevich!

Differential Revision: https://reviews.llvm.org/D30406

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@309968 91177308-0d34-0410-b5e6-96231b3b80d8