swift234

Actually fix the Makefile build for multi-file Swift libraries.

The hack to get the LLVM build system to do what we want is to define a
custom build rule for "XYZ.o" and then add "XYZ" as a dummy source file
to the SOURCES variable, which the LLVM Makefile system uses. To make it
clear that something unusual is going on here, I've changed all existing
instances of this to use "XYZ.o" in SOURCES, rather than having that name
be derived from "XYZ.swift" or whatever.

The actual Swift source files go in SWIFT_SOURCES for the time being
(and possibly forever, since Swift sources will always be built together).

Swift SVN r11058