[Scalar] Clarify the constructor from APInt and document through a test.

I want to add 512-bits support but I first want to make sure I'm
not breaking anything obvious. This is the first of a series of commit
adding tests. The first oddity found is that Scalar from APInt(s)
always constructed signed. Maybe at some point we want to revisit
this, but at least now we have a test to document how the API behaves.

<rdar://problem/46886288>

llvm-svn: 352103