Disable -Wunused-template in clangd-tidy (#7750)

This is firing on some of our _used_ templates in eval.cpp in clang-tidy
24

It was coming to `-Wall` for clang as well
(https://github.com/llvm/llvm-project/issues/202945) but was reverted
due to issues like false positives
(https://github.com/llvm/llvm-project/pull/218638). Some fixes were
applied to try enable in `-Wall` in clang 23
(https://github.com/llvm/llvm-project/pull/222336) but it still remains
disabled by default for clang.

.clangd

@@ -11,7 +11,8 @@ Diagnostics:
   #   compiler.
   # `unused-includes`: has false positives, reporting includes unused when
   #   they are used. Probably the same root cause as unused-function.
-  Suppress: [unused-function, unused-includes]
+  # `unused-template`: has false positives, which we see in eval.cpp.
+  Suppress: [unused-function, unused-includes, unused-template]
 
 ---
 

toolchain/check/eval.cpp

@@ -878,12 +878,8 @@ static auto ReplaceFieldWithConstantValue(EvalContext& eval_context,
 
 // Function template that can be called with an argument of type `T`. Used below
 // to detect which overloads of `GetConstantValue` exist.
-//
-// Marked as maybe unused at it seems the use in a requires isn't tracked by the
-// latest version of Clang's `-Wunused-template`.
-// https://github.com/llvm/llvm-project/issues/218429
 template <typename T>
-[[maybe_unused]] static auto Accept(T /*arg*/) -> void {}
+static auto Accept(T /*arg*/) -> void {}
 
 // Determines whether a `GetConstantValue` overload exists for a given ID type.
 // Note that we do not check whether `GetConstantValue` is *callable* with a