Fix perf regression in non-autolinks

Before, this was compiling the regex on every single item in the crate.

This would have been caught by `clippy::declare_interior_mutable_const`.

Author: jyn514

src/librustdoc/passes/bare_urls.rs

@@ -16,7 +16,7 @@ crate const CHECK_BARE_URLS: Pass = Pass {
     description: "detects URLs that are not hyperlinks",
 };
 
-const URL_REGEX: SyncLazy<Regex> = SyncLazy::new(|| {
+static URL_REGEX: SyncLazy<Regex> = SyncLazy::new(|| {
     Regex::new(concat!(
         r"https?://",                          // url scheme
         r"([-a-zA-Z0-9@:%._\+~#=]{2,256}\.)+", // one or more subdomains