Scalar: Use llvm integer conversion functions

StringConvert was the only non-Utility dependency of this class. Getting
rid of it means it will be easy to move this class to a lower layer.

While I was in there, I also added a couple of unit tests for the Scalar
string conversion function.

git-svn-id: https://llvm.org/svn/llvm-project/lldb/trunk@335060 91177308-0d34-0410-b5e6-96231b3b80d8
3 files changed