Fix PR16547.

We should not be asking unique_file to prepend the system temporary directory
when creating the html report. Unfortunately I don't think we can test this
with the current infrastructure since unique_file ignores MakeAbsolute if the
directory is already absolute and the paths provided by lit are.

I will take a quick look at making this api a bit less error prone.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@185707 91177308-0d34-0410-b5e6-96231b3b80d8