Improve documentation and testing of APIntFromVal

The recent unit tests we gained made clear that the semantics of APIntFromVal
are not clear, due to missing documentation. In this change we document both
the calling interface as well as the implementation of APIntFromVal. We also
make the implementation easier to read by removing the use of magic numbers.
Finally, we add tests to check the bitwidth of the created values as well as
the correct modeling of very large numbers.

Reviewed-by: Michael Kruse <llvm@meinersbur.de>
Differential Revision: https://reviews.llvm.org/D23910

llvm-svn: 279813