refactor rustdoc::invalid_html_tags tag parser

previously, this lint did not distinguish between `<img` and `<img>`,
and since the latter should be accepted under html5,
the former was also accepted.

the parser now also handles multi-line tags and multi-line attributes.

Author: lolbinarycat

src/librustdoc/lib.rs

@@ -11,6 +11,7 @@
 #![feature(file_buffered)]
 #![feature(format_args_nl)]
 #![feature(if_let_guard)]
+#![feature(iter_advance_by)]
 #![feature(iter_intersperse)]
 #![feature(round_char_boundary)]
 #![feature(rustc_private)]