Add explanation to 'existential_type_const' test

Author: Aaron1011

src/test/ui/existential_types/existential_type_const.rs

@@ -1,6 +1,10 @@
 // check-pass
 
 #![feature(existential_type)]
+// Currently, the `existential_type` feature implicitly
+// depends on `impl_trait_in_bindings` in order to work properly.
+// Specifically, this line requires `impl_trait_in_bindings` to be enabled:
+// https://github.com/rust-lang/rust/blob/481068a707679257e2a738b40987246e0420e787/src/librustc_typeck/check/mod.rs#L856
 #![feature(impl_trait_in_bindings)]
 //~^ WARN the feature `impl_trait_in_bindings` is incomplete and may cause the compiler to crash
 

src/test/ui/existential_types/existential_type_const.stderr

@@ -1,5 +1,5 @@
 warning: the feature `impl_trait_in_bindings` is incomplete and may cause the compiler to crash
-  --> $DIR/existential_type_const.rs:4:12
+  --> $DIR/existential_type_const.rs:8:12
    |
 LL | #![feature(impl_trait_in_bindings)]
    |            ^^^^^^^^^^^^^^^^^^^^^^