Fix name collision between C enum and typedef

[dtolnay]

Fixes #2008.

Example:

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:

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

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:

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

[bors-servo]

☔ The latest upstream changes (presumably f160d11) made this pull request unmergeable. Please resolve the merge conflicts.

[dtolnay]

• Rebased to resolve conflict with Add ParseCallbacks::process_comment #2347 in bindgen/codegen/mod.rs