Allow making RUSTC_BOOTSTRAP conditional on the crate name

The main change is that `UnstableOptions::from_environment` now requires
an (optional) crate name. If the crate name is unknown (`None`), then the new feature is not available and you still have to use `RUSTC_BOOTSTRAP=1`. In practice this means the feature is only available for `--crate-name`, not for `#![crate_name]`; I'm interested in supporting the second but I'm not sure how.

Other major changes:

- Added `Session::is_nightly_build()`, which uses the `crate_name` of
the session
- Added `nightly_options::match_is_nightly_build`, a convenience method
for looking up `--crate-name` from CLI arguments.
`Session::is_nightly_build()`should be preferred where possible, since
it will take into account `#![crate_name]` (I think).
- Added `unstable_features` to `rustdoc::RenderOptions`

  There is a user-facing change here: things like `RUSTC_BOOTSTRAP=0` no
  longer active nightly features. In practice this shouldn't be a big
  deal, since `RUSTC_BOOTSTRAP` is the opposite of stable and everyone
  uses `RUSTC_BOOTSTRAP=1` anyway.

- Add tests

  Check against `Cheat`, not whether nightly features are allowed.
  Nightly features are always allowed on the nightly channel.

- Only call `is_nightly_build()` once within a function

- Use booleans consistently for rustc_incremental

  Sessions can't be passed through threads, so `read_file` couldn't take a
  session. To be consistent, also take a boolean in `write_file_header`.

compiler/rustc_codegen_llvm/src/llvm_util.rs

@@ -3,7 +3,6 @@ use crate::llvm;
 use libc::c_int;
 use rustc_codegen_ssa::target_features::supported_target_features;
 use rustc_data_structures::fx::FxHashSet;
-use rustc_feature::UnstableFeatures;
 use rustc_middle::bug;
 use rustc_session::config::PrintRequest;
 use rustc_session::Session;
@@ -154,13 +153,11 @@ pub fn target_features(sess: &Session) -> Vec<Symbol> {
     let target_machine = create_informational_target_machine(sess);
     supported_target_features(sess)
         .iter()
-        .filter_map(|&(feature, gate)| {
-            if UnstableFeatures::from_environment().is_nightly_build() || gate.is_none() {
-                Some(feature)
-            } else {
-                None
-            }
-        })
+        .filter_map(
+            |&(feature, gate)| {
+                if sess.is_nightly_build() || gate.is_none() { Some(feature) } else { None }
+            },
+        )
         .filter(|feature| {
             let llvm_feature = to_llvm_feature(sess, feature);
             let cstr = CString::new(llvm_feature).unwrap();