rustdoc: Fix re-exporting primitive types

* Generate links to the primitive type docs for re-exports.
* Don't ICE on cross crate primitive type re-exports.
* Make primitive type re-exports show up cross crate.

src/test/rustdoc/auxiliary/primitive-reexport.rs

@@ -0,0 +1,8 @@
+// compile-flags: --emit metadata --crate-type lib --edition 2018
+
+#![crate_name = "foo"]
+
+pub mod bar {
+    pub use bool;
+    pub use char as my_char;
+}

src/test/rustdoc/primitive-reexport.rs

@@ -0,0 +1,28 @@
+// aux-build: primitive-reexport.rs
+// compile-flags:--extern foo --edition 2018
+
+#![crate_name = "bar"]
+
+// @has bar/p/index.html
+// @has - '//code' 'pub use bool;'
+// @has - '//code/a[@href="https://doc.rust-lang.org/nightly/std/primitive.bool.html"]' 'bool'
+// @has - '//code' 'pub use char as my_char;'
+// @has - '//code/a[@href="https://doc.rust-lang.org/nightly/std/primitive.char.html"]' 'char'
+pub mod p {
+    pub use foo::bar::*;
+}
+
+// @has bar/baz/index.html
+// @has - '//code' 'pub use bool;'
+// @has - '//code/a[@href="https://doc.rust-lang.org/nightly/std/primitive.bool.html"]' 'bool'
+// @has - '//code' 'pub use char as my_char;'
+// @has - '//code/a[@href="https://doc.rust-lang.org/nightly/std/primitive.char.html"]' 'char'
+pub use foo::bar as baz;
+
+// @has bar/index.html
+// @has - '//code' 'pub use str;'
+// @has - '//code/a[@href="https://doc.rust-lang.org/nightly/std/primitive.str.html"]' 'str'
+// @has - '//code' 'pub use i32 as my_i32;'
+// @has - '//code/a[@href="https://doc.rust-lang.org/nightly/std/primitive.i32.html"]' 'i32'
+pub use str;
+pub use i32 as my_i32;