std.Build: introduce `ConfigHeader.getOutputDir`, small refactor

`std.Build.Step.ConfigHeader` emits a *directory* containing a config
header under a given sub path, but there's no good way to actually
access that directory as a `LazyPath` in the configure phase. This is
silly; it's perfectly valid to refer to that directory, perhaps to
explicitly pass as a "-I" flag to a different toolchain invoked via a
`Step.Run`. So now, instead of the `GeneratedFile` being the actual
*file*, it should be that *directory*, i.e. `cache/o/<digest>`. We can
then easily get the *file* if needed just by using `LazyPath.path` to go
"deeper", which there is a helper function for.

The legacy `getOutput` function is now a deprecated alias for
`getOutputFile`, and `getOutputDir` is introduced.

`std.Build.Module.IncludeDir.appendZigProcessFlags` needed a fix after
this change, so I took the opportunity to refactor it a little. I was
looking at this function while working on ziglang/translate-c yesterday
and realised it could be expressed much more simply -- particularly
after the `ConfigHeader` change here.

I had to update the test `standalone/cmakedefine/` -- it turns out this
test was well and truly reaching into build system internals, and doing
horrible not-really-allowed stuff like overriding the `makeFn` of a
`TopLevelStep`. To top it all off, the test forgot to set
`b.default_step` to its "test" step, so the test never even ran. I've
refactored it to follow accepted practices and to actually, like, work.

1 files changed, 18 insertions, 32 deletions

diff --git a/lib/std/Build/Module.zig b/lib/std/Build/Module.zig
index 757f51f933..12b55a8c29 100644
--- a/lib/std/Build/Module.zig
+++ b/lib/std/Build/Module.zig
@@ -173,39 +173,25 @@ pub const IncludeDir = union(enum) {
         zig_args: *std.ArrayList([]const u8),
         asking_step: ?*Step,
     ) !void {
-        switch (include_dir) {
-            .path => |include_path| {
-                try zig_args.appendSlice(&.{ "-I", include_path.getPath2(b, asking_step) });
+        const flag: []const u8, const lazy_path: LazyPath = switch (include_dir) {
+            // zig fmt: off
+            .path                  => |lp|   .{ "-I",          lp },
+            .path_system           => |lp|   .{ "-isystem",    lp },
+            .path_after            => |lp|   .{ "-idirafter",  lp },
+            .framework_path        => |lp|   .{ "-F",          lp },
+            .framework_path_system => |lp|   .{ "-iframework", lp },
+            .config_header_step    => |ch|   .{ "-I",          ch.getOutputDir() },
+            .other_step            => |comp| .{ "-I",          comp.installed_headers_include_tree.?.getDirectory() },
+            // zig fmt: on
+            .embed_path => |lazy_path| {
+                // Special case: this is a single arg.
+                const resolved = lazy_path.getPath3(b, asking_step);
+                const arg = b.fmt("--embed-dir={}", .{resolved});
+                return zig_args.append(arg);
             },
-            .path_system => |include_path| {
-                try zig_args.appendSlice(&.{ "-isystem", include_path.getPath2(b, asking_step) });
-            },
-            .path_after => |include_path| {
-                try zig_args.appendSlice(&.{ "-idirafter", include_path.getPath2(b, asking_step) });
-            },
-            .framework_path => |include_path| {
-                try zig_args.appendSlice(&.{ "-F", include_path.getPath2(b, asking_step) });
-            },
-            .framework_path_system => |include_path| {
-                try zig_args.appendSlice(&.{ "-iframework", include_path.getPath2(b, asking_step) });
-            },
-            .other_step => |other| {
-                if (other.generated_h) |header| {
-                    try zig_args.appendSlice(&.{ "-isystem", std.fs.path.dirname(header.getPath()).? });
-                }
-                if (other.installed_headers_include_tree) |include_tree| {
-                    try zig_args.appendSlice(&.{ "-I", include_tree.generated_directory.getPath() });
-                }
-            },
-            .config_header_step => |config_header| {
-                const full_file_path = config_header.output_file.getPath();
-                const header_dir_path = full_file_path[0 .. full_file_path.len - config_header.include_path.len];
-                try zig_args.appendSlice(&.{ "-I", header_dir_path });
-            },
-            .embed_path => |embed_path| {
-                try zig_args.append(try std.mem.concat(b.allocator, u8, &.{ "--embed-dir=", embed_path.getPath2(b, asking_step) }));
-            },
-        }
+        };
+        const resolved_str = try lazy_path.getPath3(b, asking_step).toString(b.graph.arena);
+        return zig_args.appendSlice(&.{ flag, resolved_str });
     }
 };