Revert "Sema: Stop adding Windows implib link inputs for extern "..." syntax."

This reverts commit b461d07a54.

After some discussion in the team, we've decided that this is too disruptive,
especially because the linker errors are less than helpful. That's a fixable
problem, so we might reconsider this in the future, but revert it for now.

test/standalone/windows_spawn/build.zig

@@ -28,8 +28,6 @@ pub fn build(b: *std.Build) void {
         }),
     });
 
-    main.root_module.linkSystemLibrary("advapi32", .{});
-
     const run = b.addRunArtifact(main);
     run.addArtifactArg(hello);
     run.expectExitCode(0);