pw_function: Take forwarding reference instead of value

Disable unhelpful GCC warnings for GCC only.

Change-Id: I095aeaa400060a954d23b6b996d6a3d34df1a54c
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/95340
Pigweed-Auto-Submit: Wyatt Hepler <hepler@google.com>
Commit-Queue: Wyatt Hepler <hepler@google.com>
Reviewed-by: Alexei Frolov <frolv@google.com>

pw_function/public/pw_function/function.h

@@ -62,11 +62,11 @@ class Function<Return(Args...)> {
   Function& operator=(const Function&) = delete;
 
   template <typename Callable>
-  Function(Callable callable) {
+  Function(Callable&& callable) {
     if (function_internal::IsNull(callable)) {
       holder_.InitializeNullTarget();
     } else {
-      holder_.InitializeInlineTarget(std::move(callable));
+      holder_.InitializeInlineTarget(std::forward<Callable>(callable));
     }
   }
 
@@ -89,12 +89,12 @@ class Function<Return(Args...)> {
   }
 
   template <typename Callable>
-  Function& operator=(Callable callable) {
+  Function& operator=(Callable&& callable) {
     holder_.DestructTarget();
     if (function_internal::IsNull(callable)) {
       holder_.InitializeNullTarget();
     } else {
-      holder_.InitializeInlineTarget(std::move(callable));
+      holder_.InitializeInlineTarget(std::forward<Callable>(callable));
     }
     return *this;
   }

pw_function/public/pw_function/internal/function.h

@@ -38,7 +38,8 @@ struct NullEq<T, decltype(std::declval<T>() == nullptr)> {
   // nullptr. Silence this warning. (The compiler will optimize out the
   // comparison.)
   PW_MODIFY_DIAGNOSTICS_PUSH();
-  PW_MODIFY_DIAGNOSTIC(ignored, "-Waddress");
+  PW_MODIFY_DIAGNOSTIC_GCC(ignored, "-Waddress");
+  PW_MODIFY_DIAGNOSTIC_GCC(ignored, "-Wnonnull-compare");
   static constexpr bool Test(const T& v) { return v == nullptr; }
   PW_MODIFY_DIAGNOSTICS_POP();
 };