Don't encode variant visibility

[camsteffen]

See if we can save some work by not encoding variant visibility since it can be derived from its parent enum.

[rustbot]

r? @ShoyuVanilla

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[petrochenkov]

r? @petrochenkov
(I'd really prefer not to do this, even if there are small perf benefits, but in any case let's see what the benchmark will say.)

[rust-log-analyzer]

The job pr-check-2 failed! Check out the build log: (web) (plain enhanced) (plain)

Click to see the possible cause of the failure (guessed by this bot)

   --> src/bootstrap/src/core/build_steps/tool.rs:507:5
    |
394 | / macro_rules! bootstrap_tool {
395 | |     ($(
396 | |         $name:ident, $path:expr, $tool_name:expr
397 | |         $(,is_external_tool = $external:expr)*
...   |
404 | |         pub enum Tool {
    | |                  ---- variants in this enum
...   |
490 | | }
---
    | |     ^^^^^^^^^
508 | |     JsonDocLint, "src/tools/jsondoclint", "jsondoclint";
    | |     ^^^^^^^^^^^
...   |
519 | |     OptimizedDist, "src/tools/opt-dist", "opt-dist", submodules = &["src/tools/rustc-perf"];
    | |     ^^^^^^^^^^^^^
520 | |     RunMakeSupport, "src/tools/run-make-support", "run_make_support", artifact_kind = ToolArtifactKind::Library;
521 | |     IntrinsicTest, "library/stdarch/crates/intrinsic-test", "intrinsic-test";
522 | | );
    | |_- in this macro invocation

[camsteffen]

Hum this doesn't seem to possible to do without adding more than a few special cases.