Fix ICE and report a human readable error

src/test/ui/consts/min_const_fn/bad_const_fn_body_ice.rs

@@ -0,0 +1,5 @@
+const fn foo(a: i32) -> Vec<i32> {
+    vec![1, 2, 3] //~ ERROR heap allocations are not allowed in const fn
+}
+
+fn main() {}

src/test/ui/consts/min_const_fn/bad_const_fn_body_ice.stderr

@@ -0,0 +1,10 @@
+error: heap allocations are not allowed in const fn
+  --> $DIR/bad_const_fn_body_ice.rs:2:5
+   |
+LL |     vec![1, 2, 3] //~ ERROR heap allocations are not allowed in const fn
+   |     ^^^^^^^^^^^^^
+   |
+   = note: this error originates in a macro outside of the current crate (in Nightly builds, run with -Z external-macro-backtrace for more info)
+
+error: aborting due to previous error
+