Add tidy check to forbid 'CHECK: br' and 'CHECK-NOT: br' in new codegen tests

Author: zachs18

src/tools/tidy/src/style.rs

@@ -445,6 +445,7 @@ pub fn check(path: &Path, tidy_ctx: TidyCtx) {
         let mut comment_block: Option<(usize, usize)> = None;
         let is_test = file.components().any(|c| c.as_os_str() == "tests")
             || file.file_stem().unwrap() == "tests";
+        let is_codegen_test = is_test && file.components().any(|c| c.as_os_str() == "codegen-llvm");
         let is_this_file = file.ends_with(this_file) || this_file.ends_with(file);
         let is_test_for_this_file =
             is_test && file.parent().unwrap().ends_with(this_file.with_extension(""));
@@ -488,6 +489,11 @@ pub fn check(path: &Path, tidy_ctx: TidyCtx) {
                 )
             }
 
+            if is_codegen_test && trimmed.contains("CHECK") && trimmed.ends_with(": br") {
+                err("`CHECK: br` and `CHECK-NOT: br` in codegen tests are fragile to false \
+                    positives in mangled symbols. Try using `br {{.*}}` instead.")
+            }
+
             if !under_rustfmt
                 && line.chars().count() > max_columns
                 && !long_line_is_ok(&extension, is_error_code, max_columns, line)