[clang] [Clang] diagnose deleted/default redeclaration of defined friend functions (PR #136717)

[via cfe-commits]

Sirraide wrote:

Actually, I meant this of course:
 
```c++
int a() {}
struct A { friend int a() = delete; };
 
struct B { friend int b() {} };
struct C { friend int b() = delete; };
```



https://github.com/llvm/llvm-project/pull/136717