[lldb/Utility] Simplify Scalar handling of float types

Similarly to D85836, collapse all Scalar float types to a single enum
value, and use APFloat semantics to differentiate between. This
simplifies the code, and opens to door to supporting other floating
point semantics (which would be needed for fully supporting
architectures with more interesting float types such as PPC).

Differential Revision: https://reviews.llvm.org/D86220