See discussion in https://github.com/llvm/llvm-project/issues/125071.
Makes the note clearer for the unreachable case:
Before:
```
./hoge.h:5:12: warning: instantiation of function 'x<int>' required here, but no definition is available [-Wundefined-func-template]
5 | void f() { x<int>(); }
| ^
./shared_ptr2.h:4:6: note: forward declaration of template entity is here
4 | void x() { T t; (void)t; }
| ^
./hoge.h:5:12: note: add an explicit instantiation declaration to suppress this warning if 'x<int>' is explicitly instantiated in another translation unit
5 | void f() { x<int>(); }
|
```
After:
```
./hoge.h:5:12: warning: instantiation of function 'x<int>' required here, but no definition is available [-Wundefined-func-template]
5 | void f() { x<int>(); }
| ^
./shared_ptr2.h:4:6: note: declaration of template entity is unreachable here
4 | void x() { T t; (void)t; }
| ^
1 warning generated.
```
434ac4612f
