[lldb] Rewrite Scalar::Promote

This function was implementing c-like promotion rules by switching on
the both types. C promotion rules are complicated, but they are not
*that* complicated -- they basically boil down to:
- wider types trump narrower ones
- unsigned trump signed
- floating point trumps integral

With a couple of helper functions, we can rewrite the function in terms
of these rules and greatly reduce the size and complexity of this
function.