Merge bitcoin/bitcoin#30406: refactor: modernize-use-equals-default

3333bae9b2 tidy: modernize-use-equals-default (MarcoFalke)

Pull request description:

  Prior to C++20, `modernize-use-equals-default` could have been problematic because it could turn a non-aggregate into an aggregate. The risk would be that aggregate initialization would be enabled where the author did not intend to enable it.

  With C++20, aggregate for those is forbidden either way. (https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2018/p1008r1.pdf)

  So enabled it for code clarity, consistency, and possibly unlocking compiler optimizations. See https://clang.llvm.org/extra/clang-tidy/checks/modernize/use-equals-default.html

ACKs for top commit:
  stickies-v:
    ACK 3333bae9b2

Tree-SHA512: ab42ff01be7ca7e7d8b4c6a485e68426f59627d83dd827cf292304829562348dc17a52ee009f5f6f3c1c2081d7166ffac4baef23197ebeba8de7767c6ddfe255

src/util/subprocess.h

@@ -678,7 +678,7 @@ struct error
 class Buffer
 {
 public:
-  Buffer() {}
+  Buffer() = default;
   explicit Buffer(size_t cap) { buf.resize(cap); }
   void add_cap(size_t cap) { buf.resize(cap); }
 

src/util/task_runner.h

@@ -19,7 +19,7 @@ namespace util {
 class TaskRunnerInterface
 {
 public:
-    virtual ~TaskRunnerInterface() {}
+    virtual ~TaskRunnerInterface() = default;
 
     /**
      * The callback can either be queued for later/asynchronous/threaded