swift234

Add -multi-file-output mode, which generates one object file per input file.

SILGen, SIL passes, and IRGen have been factored out of main() into a
helper function "performCompile", which is called separately for each
input file. This will eventually be used to avoid rebuilding an entire
target when a single file changes.

Known issues:
- @transparent functions are not inlined across file boundaries.
  <rdar://problem/15366167>
- Some work and caches could definitely be shared between "compilations".
- Unless all files use the same LLVMContext, we get crashes.
  <rdar://problem/15283227>
- This mode is not compatible with -emit-module, so building a library still
  requires using single-file mode, or performing two invocations of swift.
  Eventually we'll have an AST-merging tool to combine the ASTs serialized
  into object files. <rdar://problem/15246467>

But the long-term design is to have these separate compilations happen in
separate cc1-like processes, so we shouldn't spend too much time optimizing
this.

Swift SVN r9889