Fix resetting the DataLayout in a Module.

No tool does this currently, but as everything else in a module we should be
able to change its DataLayout.

Most of the fix is in DataLayout to make sure it can be reset properly.

The test uses Module::setDataLayout since the fact that we mutate a DataLayout
is an implementation detail. The module could hold a OwningPtr<DataLayout> and
the DataLayout itself could be immutable.

Thanks to Philip Reames for pushing me in the right direction.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@202198 91177308-0d34-0410-b5e6-96231b3b80d8