DIBuilder: Encapsulate DIExpression's element type

`DIExpression`'s elements are 64-bit integers that are stored as
`ConstantInt`.  The accessors already encapsulate the storage.  This
commit updates the `DIBuilder` API to also encapsulate that.
DIBuilder: Encapsulate DIExpression's element type

Update for corresponding LLVM API change for
`DIBuilder::createExpression()`.