Merge #21051: Fix -Wmismatched-tags warnings

b6aadcd build: Add -Werror=mismatched-tags (Hennadii Stepanov)
1485124 Fix -Wmismatched-tags warnings (Hennadii Stepanov)

Pull request description:

  Warnings were introduced in #20749:
  ```
  ./validation.h:43:1: warning: class 'CCheckpointData' was previously declared as a struct; this is valid, but may result in linker errors under the Microsoft C++ ABI [-Wmismatched-tags]
  class CCheckpointData;
  ^
  ./chainparams.h:24:8: note: previous use is here
  struct CCheckpointData {
         ^
  ./validation.h:43:1: note: did you mean struct here?
  class CCheckpointData;
  ^~~~~
  struct
  1 warning generated.
  ```

  This change fixes AppVeyor build: https://ci.appveyor.com/project/DrahtBot/bitcoin/builds/37547435

ACKs for top commit:
  glozow:
    utACK bitcoin/bitcoin@b6aadcd 🚗
  practicalswift:
    cr ACK b6aadcd: patch looks correct

Tree-SHA512: 3ac887ebdbf9a1ae33c1fd5381b3b8d83388ad557ddeb55013acd42bb9752a5bd009e3a0eed52644a023a7a0dda1c159277981af82f58fb0abfe60b84e01bf29

Author: fanquake

configure.ac

@@ -427,6 +427,7 @@ if test "x$enable_werror" = "xyes"; then
   AX_CHECK_COMPILE_FLAG([-Werror=suggest-override],[ERROR_CXXFLAGS="$ERROR_CXXFLAGS -Werror=suggest-override"],,[[$CXXFLAG_WERROR]],
                         [AC_LANG_SOURCE([[struct A { virtual void f(); }; struct B : A { void f() final; };]])])
   AX_CHECK_COMPILE_FLAG([-Werror=unreachable-code-loop-increment],[ERROR_CXXFLAGS="$ERROR_CXXFLAGS -Werror=unreachable-code-loop-increment"],,[[$CXXFLAG_WERROR]])
+  AX_CHECK_COMPILE_FLAG([-Werror=mismatched-tags], [ERROR_CXXFLAGS="$ERROR_CXXFLAGS -Werror=mismatched-tags"], [], [$CXXFLAG_WERROR])
 fi
 
 if test "x$CXXFLAGS_overridden" = "xno"; then

src/validation.h

@@ -40,7 +40,7 @@ class CBlockIndex;
 class CBlockTreeDB;
 class CBlockUndo;
 class CChainParams;
-class CCheckpointData;
+struct CCheckpointData;
 class CInv;
 class CConnman;
 class CScriptCheck;