llvm::InitLLVM starts getting stacks on assertion errors. Tieing errs to outs causes outs to be flushed when errs is used (avoiding munging out incompletely flushed output).
1) toolchain and explorer use the same working dir logic, share it
2) explorer's carbon.cpp and main_bin.cpp use the same relative path logic, share it
3) You can append a longer path in one call with the right kind of iterator
4) Fix what's maybe a bug in passing `relative_prelude_path.str()` to `cl::init`
5) Collapse Main and ExplorerMain to avoid passing more parameters between
This should clean up our top level directory and the build patterns.
No non-mechanical edits here. Just injecting `toolchain/` and
`TOOLCHAIN_` and then running formatting tools.
Jon Ross-Perkins
Chandler Carruth