Merge bitcoin/bitcoin#28999: build: Enable -Wunreachable-code

fa8adbe build: Enable -Wunreachable-code (MarcoFalke)

Pull request description:

  It seems a bit confusing to write code after a `return`. This can even lead to bugs, or incorrect code, such as https://github.com/bitcoin/bitcoin/pull/28830/files#r1415372320 . (Edit: The linked instance is not found by clang's `-Wunreachable-code`).

  Fix all issues by enabling `-Wunreachable-code`.

  This flag also enables `-Wunreachable-code-loop-increment`, according to https://clang.llvm.org/docs/DiagnosticsReference.html#wunreachable-code, so remove that.

ACKs for top commit:
  ajtowns:
    > ACK [fa8adbe](bitcoin/bitcoin@fa8adbe)
  stickies-v:
    ACK fa8adbe
  jonatack:
    ACK fa8adbe tested with arm64 clang 17.0.6

Tree-SHA512: 12a2f74b69ae002e62ae08038f7458837090a12051a4c154d05ae4bb26fb19fc1fa76c63aedf2b3fbb36f048c593ca3b8c0efe03fe93cf07a0fd114fc84ce1e7

Author: fanquake

configure.ac

@@ -436,8 +436,8 @@ if test "$CXXFLAGS_overridden" = "no"; then
   AX_CHECK_COMPILE_FLAG([-Wlogical-op], [WARN_CXXFLAGS="$WARN_CXXFLAGS -Wlogical-op"], [], [$CXXFLAG_WERROR])
   AX_CHECK_COMPILE_FLAG([-Woverloaded-virtual], [WARN_CXXFLAGS="$WARN_CXXFLAGS -Woverloaded-virtual"], [], [$CXXFLAG_WERROR])
   AX_CHECK_COMPILE_FLAG([-Wsuggest-override], [WARN_CXXFLAGS="$WARN_CXXFLAGS -Wsuggest-override"], [], [$CXXFLAG_WERROR])
-  AX_CHECK_COMPILE_FLAG([-Wunreachable-code-loop-increment], [WARN_CXXFLAGS="$WARN_CXXFLAGS -Wunreachable-code-loop-increment"], [], [$CXXFLAG_WERROR])
   AX_CHECK_COMPILE_FLAG([-Wimplicit-fallthrough], [WARN_CXXFLAGS="$WARN_CXXFLAGS -Wimplicit-fallthrough"], [], [$CXXFLAG_WERROR])
+  AX_CHECK_COMPILE_FLAG([-Wunreachable-code], [WARN_CXXFLAGS="$WARN_CXXFLAGS -Wunreachable-code"], [], [$CXXFLAG_WERROR])
 
   if test "$suppress_external_warnings" != "no" ; then
     AX_CHECK_COMPILE_FLAG([-Wdocumentation], [WARN_CXXFLAGS="$WARN_CXXFLAGS -Wdocumentation"], [], [$CXXFLAG_WERROR])

src/wallet/walletdb.cpp

@@ -1499,17 +1499,19 @@ std::unique_ptr<WalletDatabase> MakeDatabase(const fs::path& path, const Databas
     if (format == DatabaseFormat::SQLITE) {
 #ifdef USE_SQLITE
         return MakeSQLiteDatabase(path, options, status, error);
-#endif
+#else
         error = Untranslated(strprintf("Failed to open database path '%s'. Build does not support SQLite database format.", fs::PathToString(path)));
         status = DatabaseStatus::FAILED_BAD_FORMAT;
         return nullptr;
+#endif
     }
 
 #ifdef USE_BDB
     return MakeBerkeleyDatabase(path, options, status, error);
-#endif
+#else
     error = Untranslated(strprintf("Failed to open database path '%s'. Build does not support Berkeley DB database format.", fs::PathToString(path)));
     status = DatabaseStatus::FAILED_BAD_FORMAT;
     return nullptr;
+#endif
 }
 } // namespace wallet