[Modules] Move the LeakDetector header into the IR library where the
source file had already been moved. Also move the unittest into the IR
unittest library.

This may seem an odd thing to put in the IR library but we only really
use this with instructions and it needs the LLVM context to work, so it
is intrinsically tied to the IR library.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@202842 91177308-0d34-0410-b5e6-96231b3b80d8