[lld-macho][nfc] Move list of section names into InputSection.h

They were previously in SyntheticSections.h, but now there are
a bunch of non-synthetic section names in the list.

Also renamed `__functionStarts` to `__func_starts` for uniformity with
other section names + keeps the name under 16 characters (in case we ever
want to write it out as a real section).

Reviewed By: #lld-macho, compnerd

Differential Revision: https://reviews.llvm.org/D98586