swift234

[SIL] Use the ReturnLocation/ImplicitReturnLocation instead of generic SILLocations.

We mark the branch instructions leading into single epilog code with ReturnLocation/ImplicitReturnLocation. If SIL Gen simplifies the code and merges the code representing the return into the epilog block, the terminator of the epilog block (the ReturnInst) will have the return location info on it. Otherwise, the ReturnInst has the RegularLocation, which represents the enclosing FunctionExpr or Constructor/Destructor Decls.
(I've discussed dropping the optimization from SILGen, and keeping the epilog code canonical, with Adrian; but he said that there might not be any wins in doing so, so keeping it for now.)

Added AutoGeneratedLocation to represent segments of code generated by SILGen. This will be used for thunks and other auto-generated segments.

Swift SVN r7634