coverage: llvm-cov expects column numbers to be bytes, not code points

compiler/rustc_mir_transform/src/lib.rs

@@ -9,6 +9,7 @@
 #![feature(min_specialization)]
 #![feature(never_type)]
 #![feature(option_get_or_insert_default)]
+#![feature(round_char_boundary)]
 #![feature(trusted_step)]
 #![feature(try_blocks)]
 #![feature(yeet_expr)]