Feature gate and make must_not_suspend allow-by-default

This lint is not yet ready for stable use, primarily due to false positives in edge
cases; we want to test it out more before stabilizing.

compiler/rustc_lint/src/lib.rs

@@ -300,7 +300,6 @@ fn register_builtins(store: &mut LintStore, no_interleave_lints: bool) {
         UNUSED_LABELS,
         UNUSED_PARENS,
         UNUSED_BRACES,
-        MUST_NOT_SUSPEND,
         REDUNDANT_SEMICOLONS
     );