Eliminate the SessionGlobals from librustc_ast.

By moving `{known,used}_attrs` from `SessionGlobals` to `Session`. This
means they are accessed via the `Session`, rather than via TLS. A few
`Attr` methods and `librustc_ast` functions are now methods of
`Session`.

All of this required passing a `Session` to lots of functions that didn't
already have one. Some of these functions also had arguments removed, because
those arguments could be accessed directly via the `Session` argument.

`contains_feature_attr()` was dead, and is removed.

Some functions were moved from `librustc_ast` elsewhere because they now need
to access `Session`, which isn't available in that crate.
- `entry_point_type()` --> `librustc_builtin_macros`
- `global_allocator_spans()` --> `librustc_metadata`
- `is_proc_macro_attr()` --> `Session`

Author: nnethercote

Cargo.lock

@@ -3195,7 +3195,6 @@ dependencies = [
  "rustc_macros",
  "rustc_serialize",
  "rustc_span",
- "scoped-tls",
  "smallvec 1.4.0",
  "tracing",
 ]
@@ -3467,6 +3466,7 @@ dependencies = [
  "rustc_index",
  "rustc_macros",
  "rustc_serialize",
+ "rustc_session",
  "rustc_span",
  "rustc_target",
  "smallvec 1.4.0",

src/librustc_ast/Cargo.toml

@@ -12,7 +12,6 @@ doctest = false
 [dependencies]
 rustc_serialize = { path = "../librustc_serialize" }
 log = { package = "tracing", version = "0.1" }
-scoped-tls = "1.0"
 rustc_span = { path = "../librustc_span" }
 rustc_data_structures = { path = "../librustc_data_structures" }
 rustc_index = { path = "../librustc_index" }