Move is_min_const_fn query to librustc_mir.

The only two uses of the associated methods are in librustc_mir and
librustdoc. Please tell me if there is a better choice.

src/librustdoc/lib.rs

@@ -32,6 +32,7 @@ extern crate rustc_interface;
 extern crate rustc_lexer;
 extern crate rustc_lint;
 extern crate rustc_metadata;
+extern crate rustc_mir;
 extern crate rustc_parse;
 extern crate rustc_resolve;
 extern crate rustc_span as rustc_span;