Rather than requiring macro implementations to add required whitespace
and indentation, basic format all macro expansions. Right now this uses
the default four space indentation, we can consider having that inferred
later. Macros can opt-out of automatic formatting by implementing
`formatMode` and setting it to `.disabled`.
Also moves the extra newlines before/after expansions to a new "Inline
Macro" refactoring.
Resolves rdar://107731047.
