Rename langref.rst to ir.rst.

This aligns with our use of the term "IR" rather than "IL" or
other terms involving "language".

lib/codegen/src/result.rs

@@ -19,7 +19,7 @@ pub enum CodegenError {
     /// Cranelift can compile very large and complicated functions, but the [implementation has
     /// limits][limits] that cause compilation to fail when they are exceeded.
     ///
-    /// [limits]: https://cranelift.readthedocs.io/en/latest/langref.html#implementation-limits
+    /// [limits]: https://cranelift.readthedocs.io/en/latest/ir.html#implementation-limits
     #[fail(display = "Implementation limit exceeded")]
     ImplLimitExceeded,