This is just to be able to directly run the driver (e.g., `bazel run
//toolchain/driver:carbon compile foo.carbon`); it shouldn't affect
anything else.
This handles toolchain failures per-file. The intent is to allow placing
both "success" and "fail" tests in the same file, using splits. However,
this PR only adds support and updates existing tests to continue
passing.
Continuing along with #3070. Note this is just a file rename, with BUILD
edits; every file previously in semantics/ should show as moved (except
maybe BUILDs, which split).
Jon Ross-Perkins