Fix name collision between C enum and typedef (#2326)

Fixes #2008.

Example:

```c
enum Enum { Variant };
typedef int16_t Enum;
```

This is valid and idiomatic C (though not valid C++). `cbindgen` uses this idiom as the default C translation of Rust enums, the equivalent of what would be `enum Enum : int16_t { Variant };` in C++.

`bindgen header.h` before:

```rust
pub const Enum_Variant: Enum = 0;
pub type Enum = ::std::os::raw::c_uint;
pub type Enum = i16;
```

```console
error[E0428]: the name `Enum` is defined multiple times
 --> generated.rs:3:1
  |
2 | pub type Enum = ::std::os::raw::c_uint;
  | --------------------------------------- previous definition of the type `Enum` here
3 | pub type Enum = i16;
  | ^^^^^^^^^^^^^^^^^^^^ `Enum` redefined here
  |
  = note: `Enum` must be defined only once in the type namespace of this module
```

After:

```rust

pub const Enum_Variant: Enum = 0;
pub type Enum = i16;
```

1 files changed, 10 insertions, 5 deletions

diff --git a/bindgen/codegen/mod.rs b/bindgen/codegen/mod.rs
index b4050637..154d7fd1 100644
--- a/bindgen/codegen/mod.rs
+++ b/bindgen/codegen/mod.rs
@@ -2709,6 +2709,7 @@ impl<'a> EnumBuilder<'a> {
         mut attrs: Vec<proc_macro2::TokenStream>,
         repr: proc_macro2::TokenStream,
         enum_variation: EnumVariation,
+        has_typedef: bool,
     ) -> Self {
         let ident = Ident::new(name, Span::call_site());
 
@@ -2741,10 +2742,12 @@ impl<'a> EnumBuilder<'a> {
             EnumVariation::Consts => {
                 let mut variants = Vec::new();
 
-                variants.push(quote! {
-                    #( #attrs )*
-                    pub type #ident = #repr;
-                });
+                if !has_typedef {
+                    variants.push(quote! {
+                        #( #attrs )*
+                        pub type #ident = #repr;
+                    });
+                }
 
                 EnumBuilder::Consts { variants }
             }
@@ -3157,8 +3160,10 @@ impl CodeGenerator for Enum {
         }
 
         let repr = repr.to_rust_ty_or_opaque(ctx, item);
+        let has_typedef = ctx.is_enum_typedef_combo(item.id());
 
-        let mut builder = EnumBuilder::new(&name, attrs, repr, variation);
+        let mut builder =
+            EnumBuilder::new(&name, attrs, repr, variation, has_typedef);
 
         // A map where we keep a value -> variant relation.
         let mut seen_values = HashMap::<_, Ident>::default();