Plumbing this through to the inliner necessitated the creation of a
SILOptions class (like FrontendOptions and IRGenOptions). I'll move
more things into this soon.
One change: for compatibility with the new driver, the option must be
specified as "-sil-inline-threshold 50" instead of "-sil-inline-threshold=50".
(We're really trying to be consistent about joined-equals vs. separate
in the new frontend.)
Swift SVN r13193
