swift234

Refactor SILLinkerVisitor into its own local files, Linker.{h,cpp}. This is still hidden in the SIL library.

This is only used by SILModule but is not integral to a SILModule so it makes
sense to have it in its own file. It keeps SILModule.cpp more focused. We still
keep it in a private header though since it is only meant to be used by
SILModule.cpp.

Swift SVN r25985