stage2: make --color on affect progress bar too

Before, --color on would affect colored compile error printing but not
affect terminal progress bar printing. It was intended for this option
to affect both; now it does.

This causes a failure when building the language reference, which
contains code for parsing terminal output and rendering HTML. Now it
must be expanded to handle 'K' and 'D' codes to simulate a terminal
cursor moving, and the CI will fail until that capability is added in a
later commit of this branch.

I extracted this change from ziglang#13560 so that the idea is not lost but we
can solve this issue separately.

Author: andrewrk

src/Compilation.zig

@@ -2355,7 +2355,16 @@ pub fn update(comp: *Compilation) !void {
     var progress: std.Progress = .{ .dont_print_on_dumb = true };
     const main_progress_node = progress.start("", 0);
     defer main_progress_node.end();
-    if (comp.color == .off) progress.terminal = null;
+    switch (comp.color) {
+        .off => {
+            progress.terminal = null;
+        },
+        .on => {
+            progress.terminal = std.io.getStdErr();
+            progress.supports_ansi_escape_codes = true;
+        },
+        .auto => {},
+    }
 
     try comp.performAllTheWork(main_progress_node);