Add ui test for ensuring that users cannot use literal-escaper crate for the time being

Author: GuillaumeGomez

tests/ui/feature-gates/literal-escaper.rs

@@ -0,0 +1,3 @@
+#![crate_type = "lib"]
+
+extern crate literal_escaper; //~ ERROR

tests/ui/feature-gates/literal-escaper.stderr

@@ -0,0 +1,13 @@
+error[E0658]: use of unstable library feature `rustc_private`: this crate is being loaded from the sysroot, an unstable location; did you mean to load this crate from crates.io via `Cargo.toml` instead?
+  --> $DIR/literal-escaper.rs:3:1
+   |
+LL | extern crate literal_escaper;
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: see issue #27812 <https://github.com/rust-lang/rust/issues/27812> for more information
+   = help: add `#![feature(rustc_private)]` to the crate attributes to enable
+   = note: this compiler was built on YYYY-MM-DD; consider upgrading it if it is out of date
+
+error: aborting due to 1 previous error
+
+For more information about this error, try `rustc --explain E0658`.