[Testing/Support] Remove the const_cast in TakeExpected

Summary:
The const_cast in the "const" version of TakeExpected was quite
dangerous, as the function does indeed modify the apparently const
argument.

I assume the reason the const overload was added was to make the
function bind to xvalues(temporaries). That can be also achieved with
rvalue references, so I use that instead.

Using the ASSERT macros on const Expected objects will now become
illegal, but I believe that is correct, as it is not actually possible
to inspect the error stored in an Expected object without modifying it.

Reviewers: zturner, chandlerc

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D34405

llvm-svn: 306001