[lldb/Utility] Simplify Scalar::PromoteToMaxType

The function had very complicated signature, because it was trying to
avoid making unnecessary copies of the Scalar object. However, this
class is not hot enough to worry about these kinds of optimizations. My
making copies unconditionally, we can simplify the function and all of
its call sites.

Differential Revision: https://reviews.llvm.org/D85906