Add has_default to GenericParamDefKind::Const

This currently creates a field which is always false on GenericParamDefKind for future use when
consts are permitted to have defaults

Update const_generics:default locations

Previously just ignored them, now actually do something about them.

Fix using type check instead of value

Add parsing

This adds all the necessary changes to lower const-generics defaults from parsing.

Change P<Expr> to AnonConst

This matches the arguments passed to instantiations of const generics, and makes it specific to
just anonymous constants.

Attempt to fix lowering bugs

src/test/ui/const-generics/defaults/const-default.rs

@@ -0,0 +1,15 @@
+// check-pass
+
+#![feature(const_generics)]
+#![feature(const_generic_defaults)]
+#![allow(incomplete_features)]
+
+
+#[derive(Default)]
+pub struct ConstDefault<const N: usize = 3> {
+  items: [u32; N]
+}
+
+pub fn main() {
+  let s = ConstDefault::default();
+}

src/test/ui/const-generics/min_const_generics/default_function_param.rs

@@ -1,3 +1,6 @@
+#![feature(const_generic_defaults)]
+#![feature(min_const_generics)]
+
 fn foo<const SIZE: usize = 5>() {}
 //~^ ERROR default values for const generic parameters are experimental
 

src/test/ui/const-generics/min_const_generics/default_trait_param.rs

@@ -1,4 +1,2 @@
 trait Foo<const KIND: bool = true> {}
 //~^ ERROR default values for const generic parameters are experimental
-
-fn main() {}

src/test/ui/feature-gate/feature-gate-const_generic_defaults.rs

@@ -0,0 +1,5 @@
+#![feature(min_const_generics)]
+#![crate_type="lib"]
+
+struct A<const N: usize = 3>;
+//~^ ERROR default values for

src/test/ui/feature-gate/feature-gate-const_generic_defaults.stderr

@@ -0,0 +1,10 @@
+error: default values for const generic parameters are unstable
+  --> $DIR/feature-gate-const_generic_defaults.rs:4:27
+   |
+LL | struct A<const N: usize = 3>;
+   |                           ^
+   |
+   = note: to enable them use #![feature(const_generic_defaults)]
+
+error: aborting due to previous error
+