swift234

[sil-dead-function-elimination] Re-factoring of the external function definitions elimination pass.

Rename LateDeadFunctionElimination into ExternalFunctionDefinitionsElimination.
Move ExternalFunctionDefinitionsElimination  out of DeadFunctionElimination and make it a separate pass.
Move a common logic shared by  DeadFunctionElimination and ExternalFunctionDefinitionsElimination into a newly created base class.

Make ExternalFunctionDefinitionsElimination pass eliminate just those external functions which are only reachable via vtables and witness_tables, but leave directly reachable function definitions in place. This gives LLVM more chances to analyze directly reachable functions for side-effects and perform better optimizations based on this. Once we have a proper IPO support for Swift, we can eliminate all external function definitions, including directly reachable ones, as there will be other ways to get information about their side-effects.

Swift SVN r24546