Duplicate warning diagnostic produced by compiler for same code

Consider the following reduced test case originally derived from `clang/test/SemaTemplate/instantiate-member-initializers.cpp`:
```c++
template void C2() { ""; }
template void C2< int >();
```

When compiled, the compiler produces the warning diagnostic twice
https://godbolt.org/z/McYKrb7zj:
```

clang++: warning: argument unused during compilation: '-c' [-Wunused-command-line-argument]
:1:30: warning: expression result unused [-Wunused-value]
1 | template void C2() { ""; }
| ^~
:1:30: warning: expression result unused [-Wunused-value]
1 | template void C2() { ""; }
| ^~
:2:15: note: in instantiation of function template specialization 'C2' requested here
2 | template void C2< int >();
| ^
2 warnings generated.
Compiler returned: 0
```

Start by reproducing the reduced C++ case and compare it with clang/test/SemaTemplate/instantiate-member-initializers.cpp. Trace the diagnostic emitted for the unused expression during explicit function-template instantiation, then update or add the relevant test so the warning appears only once while the instantiation note remains correct.

Written by the indexing model from the issue text.