logging: Add preprocessor workaround for MSVC

Needed to fix errors like:

const Source &_LogSource(const Source &)': expects 1 arguments - 3 provided
const Source &_LogSource(const Source &)': expects 1 arguments - 3 provided

due to a compiler bug:

https://stackoverflow.com/questions/5134523/msvc-doesnt-expand-va-args-correctly/5134656#5134656

Example CI failure:

https://github.com/bitcoin/bitcoin/actions/runs/8072891468/job/22055528830?pr=29256

src/logging.h

@@ -299,18 +299,21 @@ void _LogFormat(LogFn&& log, Source&& source, util::ConstevalFormatString<sizeof
     log(source, source.Format(fmt, args...));
 }
 
-//! Internal helper to return first arg in a __VA_ARGS__ pack.
-#define _FirstArg(arg, ...) arg
+//! Internal helper to return first arg in a __VA_ARGS__ pack. This could be
+//! simplified to `#define _FirstArg(arg, ...) arg` if not for a preprocessor
+//! bug in Visual C++.
+#define _FirstArgImpl(arg, ...) arg
+#define _FirstArg(args) _FirstArgImpl args
 
 //! Internal helper to check level and log. Avoids evaluating arguments if not logging.
 #define _LogPrint(level, ...)                                             \
     do {                                                                  \
-        if (LogEnabled(_LogSource(_FirstArg(__VA_ARGS__)), (level))) {    \
+        if (LogEnabled(_LogSource(_FirstArg((__VA_ARGS__))), (level))) {  \
             const auto& func = __func__;                                  \
             _LogFormat([&](auto&& source, auto&& message) {               \
                 source.logger.LogPrintStr(message, func, __FILE__,        \
                     __LINE__, source.category, (level));                  \
-            }, _LogSource(_FirstArg(__VA_ARGS__)), __VA_ARGS__);          \
+            }, _LogSource(_FirstArg((__VA_ARGS__))), __VA_ARGS__);        \
         }                                                                 \
     } while (0)