Treat nightlies for a version as complete

This commit makes cfg(version) treat the nightlies
for version 1.n.0 as 1.n.0, even though that nightly
version might not have all stabilizations and features
of the released 1.n.0. This is done for greater
convenience for people who want to test a newly
stabilized feature on nightly.

For users who wish to pin nightlies, this commit adds
a -Z assume-incomplete-release option that they can
enable if there are any issues due to this change.

compiler/rustc_interface/src/tests.rs

@@ -538,6 +538,7 @@ fn test_debugging_options_tracking_hash() {
     // This list is in alphabetical order.
     tracked!(allow_features, Some(vec![String::from("lang_items")]));
     tracked!(always_encode_mir, true);
+    tracked!(assume_incomplete_release, true);
     tracked!(asm_comments, true);
     tracked!(binary_dep_depinfo, true);
     tracked!(chalk, true);