[ORC] Add more utilities to aid debugging output.

(1) A const accessor for the LLVMContext held by a ThreadSafeContext.

(2) A const accessor for the ThreadSafeModules held by an IRMaterializationUnit.

(3) A const MaterializationResponsibility reference to IRTransformLayer2's
    transform function. This makes IRTransformLayer2 useful for JIT debugging
    (since it can inspect JIT state through the responsibility argument) as well
    as program transformations.

llvm-svn: 343365