[lld][ELF] Assign sectionChoice properly to ELF atoms

This sets the sectionChoice property for DefinedAtoms. The output section name
is derived by the property of the atom. This also decreases native file size.

Adds a test.
Move SCEVAffinator member definitions out of class body

Instead of defining the relevant functions inline, we now just keep the
declarations in the class itself. This makes the class declaration a lot
easier to read as all functions can be seen at once. We also use this
opportunity to privatize all functions not used in the public interface of the
class.
clang-format recent change