This was never being compiled at all and was bitrotting
as a result.

Also compile SampleAnalyzerPlugin as a module, not a library,
and fix a mistake with not passing the source files
to add_clang_library().

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@150378 91177308-0d34-0410-b5e6-96231b3b80d8