Handle the const struct * and struct * patterns (#2304)

Given that C keeps a different namespace for `struct`/`enum`/`union` and `typedef` aliases. The
following patterns
```c
typedef const struct foo {
    void *inner;
} *foo;

typedef struct bar {
    void *inner;
} *bar;
```
are valid C code and produces both a `struct` and a pointer called `foo`
and `bar` in different namespaces. Given that Rust does not make this
distinction, we add the `_ptr` prefix to the pointer type aliases to
avoid any name collisions.

Author: pvdrz

bindgen-tests/tests/expectations/tests/typedef-pointer-overlap.rs

@@ -0,0 +1,30 @@
+typedef const struct foo {
+    char inner;
+} *foo;
+
+typedef struct bar {
+    char inner;
+} *bar;
+
+typedef struct baz *baz;
+
+typedef union cat {
+    int standard_issue;
+} *cat;
+
+typedef enum mad { scientist } *mad;
+
+void takes_foo_ptr(foo);
+void takes_foo_struct(struct foo);
+
+void takes_bar_ptr(bar);
+void takes_bar_struct(struct bar);
+
+void takes_baz_ptr(baz);
+void takes_baz_struct(struct baz);
+
+void takes_cat_ptr(cat);
+void takes_cat_union(union cat);
+
+void takes_mad_ptr(mad);
+void takes_mad_enum(enum mad);

bindgen-tests/tests/headers/typedef-pointer-overlap.h

@@ -1094,17 +1094,32 @@ impl Type {
                 }
                 CXType_Typedef => {
                     let inner = cursor.typedef_type().expect("Not valid Type?");
-                    let inner =
+                    let inner_id =
                         Item::from_ty_or_ref(inner, location, None, ctx);
-                    if inner == potential_id {
+                    if inner_id == potential_id {
                         warn!(
                             "Generating oqaque type instead of self-referential \
                             typedef");
                         // This can happen if we bail out of recursive situations
                         // within the clang parsing.
                         TypeKind::Opaque
                     } else {
-                        TypeKind::Alias(inner)
+                        // Check if this type definition is an alias to a pointer of a `struct` /
+                        // `union` / `enum` with the same name and add the `_ptr` suffix to it to
+                        // avoid name collisions.
+                        if let Some(ref mut name) = name {
+                            if inner.kind() == CXType_Pointer &&
+                                !ctx.options().c_naming
+                            {
+                                let pointee = inner.pointee_type().unwrap();
+                                if pointee.kind() == CXType_Elaborated &&
+                                    pointee.declaration().spelling() == *name
+                                {
+                                    *name += "_ptr";
+                                }
+                            }
+                        }
+                        TypeKind::Alias(inner_id)
                     }
                 }
                 CXType_Enum => {