Enable incremental independent of stage

Previously we'd only do so for stage 0 but with keep-stage
improvements it seems likely that we'll see more developers working in
the stage 1, so we should allow enabling incremental for them.

Ideally, the check we probably want is to only enable incremental for
the last compiler build scheduled, but there's no good way to do so
today. Just enabling incremental in all stages should be sufficient;
we may be doing extra work that's needles -- compiling incrementally
something that will never be recompiled in-place -- but that should be
sufficiently unlikely (i.e., users either don't care or won't be
compiling the compiler twice).

src/bootstrap/builder.rs

@@ -903,10 +903,7 @@ impl<'a> Builder<'a> {
                 .env("RUSTC_SNAPSHOT_LIBDIR", self.rustc_libdir(compiler));
         }
 
-        // Ignore incremental modes except for stage0, since we're
-        // not guaranteeing correctness across builds if the compiler
-        // is changing under your feet.`
-        if self.config.incremental && compiler.stage == 0 {
+        if self.config.incremental {
             cargo.env("CARGO_INCREMENTAL", "1");
         }