Add ignore-tidy-filelength on ty/context

This is so we avoid a massive break of other people's code. Gonna run
rustfmt and split the file on a different PR.

src/librustc/ty/context.rs

@@ -1,3 +1,4 @@
+// ignore-tidy-filelength
 //! Type context book-keeping.
 
 use crate::arena::Arena;