[analyzer] [tests] Remove empty folders in reference results, do not store diffs.txt

[analyzer] [tests] Remove empty folders in reference results, do not store diffs.txt

Storing diffs.txt is now redundant, as we simply dump the CmpRuns output
to stdout (it is saved in CI and tends to be small).
Not generating those files enables us to remove empty folders, which
confuse git, as it would not add them with reference results.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@316948 91177308-0d34-0410-b5e6-96231b3b80d8