scan-build: Add a --keep-empty option for better testing.

SATestBuild expects to compare output directories for each invocation of
scan-build that it runs, but scan-build clears out empty directories by
default. We were coincidentally not getting that behavior until r173294.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@173383 91177308-0d34-0410-b5e6-96231b3b80d8