CompilationDatabase: Sure-up ownership of compilation databases using std::unique_ptr

Diving into the memory leaks fixed by r213851 there was one case of a
memory leak of a CompilationDatabase due to not properly taking
ownership of the result of "CompilationDatabase::autoDetectFromSource".
Given that both implementations and callers have been using unique_ptr
to own CompilationDatabase objects - make this explicit in the API to
reduce the risk of further leaks.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@215215 91177308-0d34-0410-b5e6-96231b3b80d8