Limit the size of cgu names when using the -Zhuman-readable-cgu-names option

Prior to this change, cgu names could be generated which would result in
filenames longer than the limit imposed by the OS.

compiler/rustc_middle/src/lib.rs

@@ -51,6 +51,7 @@
 #![feature(negative_impls)]
 #![feature(never_type)]
 #![feature(ptr_alignment_type)]
+#![feature(round_char_boundary)]
 #![feature(rustc_attrs)]
 #![feature(rustdoc_internals)]
 #![feature(trusted_len)]