ty: projections in transparent_newtype_field

This commit modifies `transparent_newtype_field` so that it handles
projections with generic parameters, where `normalize_erasing_regions`
would ICE.

Signed-off-by: David Wood <david@davidtw.co>

src/test/ui/lint/lint-ctypes-73249-1.rs

@@ -0,0 +1,21 @@
+// check-pass
+#![deny(improper_ctypes)]
+
+pub trait Foo {
+    type Assoc: 'static;
+}
+
+impl Foo for () {
+    type Assoc = u32;
+}
+
+extern "C" {
+    pub fn lint_me(x: Bar<()>);
+}
+
+#[repr(transparent)]
+pub struct Bar<T: Foo> {
+    value: &'static <T as Foo>::Assoc,
+}
+
+fn main() {}

src/test/ui/lint/lint-ctypes-73249.rs

@@ -0,0 +1,21 @@
+// check-pass
+#![deny(improper_ctypes)]
+
+pub trait Foo {
+    type Assoc;
+}
+
+impl Foo for () {
+    type Assoc = u32;
+}
+
+extern "C" {
+    pub fn lint_me(x: Bar<()>);
+}
+
+#[repr(transparent)]
+pub struct Bar<T: Foo> {
+    value: <T as Foo>::Assoc,
+}
+
+fn main() {}