[clang] Reland "[Clang] Enable -Wunused-template under -Wall" (PR #208001)

[Jan André Reuter via cfe-commits]

Thyre wrote:

If I saw correctly, it looks like `offload` fails to build in pull requests with this change being re-landed. Failing CI runs include e.g.:
- https://github.com/llvm/llvm-project/actions/runs/29019670314/job/86122960185?pr=208455
- https://github.com/llvm/llvm-project/actions/runs/28980410612/job/85997632273?pr=208122

Example:
```
2026-07-08T22:43:55.4409871Z /home/gha/actions-runner/_work/llvm-project/llvm-project/offload/plugins-nextgen/amdgpu/src/rtl.cpp:129:21: error: unused function template 'iterate' [-Werror,-Wunused-template]
2026-07-08T22:43:55.4410701Z   129 | static hsa_status_t iterate(IterFuncTy Func, IterFuncArgTy FuncArg,
2026-07-08T22:43:55.4411080Z       |                     ^~~~~~~
2026-07-08T22:43:55.4411328Z 1 error generated.
[...]
2026-07-08T22:43:55.6791199Z In file included from /home/gha/actions-runner/_work/llvm-project/llvm-project/offload/liboffload/src/OffloadImpl.cpp:14:
2026-07-08T22:43:55.6791997Z In file included from /home/gha/actions-runner/_work/llvm-project/llvm-project/offload/liboffload/include/OffloadImpl.hpp:10:
2026-07-08T22:43:55.6793090Z /home/gha/actions-runner/_work/llvm-project/llvm-project/offload/liboffload/../plugins-nextgen/common/include/PluginInterface.h:101:14: error: unused function template 'check' [-Werror,-Wunused-template]
2026-07-08T22:43:55.6794035Z   101 | static Error check(int32_t ErrorCode, const char *ErrFmt, ArgsTy... Args);
2026-07-08T22:43:55.6794421Z       |              ^~~~~
2026-07-08T22:43:55.6794646Z 1 error generated.
[...]
2026-07-08T22:43:58.9306604Z In file included from /home/gha/actions-runner/_work/llvm-project/llvm-project/offload/plugins-nextgen/common/src/JIT.cpp:16:
2026-07-08T22:43:58.9307673Z /home/gha/actions-runner/_work/llvm-project/llvm-project/offload/plugins-nextgen/common/include/PluginInterface.h:101:14: error: unused function template 'check' [-Werror,-Wunused-template]
2026-07-08T22:43:58.9308623Z   101 | static Error check(int32_t ErrorCode, const char *ErrFmt, ArgsTy... Args);
2026-07-08T22:43:58.9309009Z       |              ^~~~~
2026-07-08T22:43:58.9309239Z 1 error generated.


2026-07-09T13:01:49.3420368Z /home/gha/actions-runner/_work/llvm-project/llvm-project/offload/plugins-nextgen/common/include/OffloadError.h:52:20: error: unused function template 'createOffloadError' [-Werror,-Wunused-template]
2026-07-09T13:01:49.3421724Z    52 | static llvm::Error createOffloadError(error::ErrorCode Code, const char *ErrFmt,
2026-07-09T13:01:49.3422610Z       |                    ^~~~~~~~~~~~~~~~~~
2026-07-09T13:01:49.3422966Z 1 error generated.
```

https://github.com/llvm/llvm-project/pull/208001