Fix leak in ObjCSelfInitChecker, found by LSan.

BugReport doesn't take ownership of the bug type, so let the checker own the
the bug type.  (Requires making the bug type mutable, which is icky, but which
is also what other checkers do.)


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@208110 91177308-0d34-0410-b5e6-96231b3b80d8