Change KJ_LOG macro to use "for" statement

Using "for" statement instead of "if", as suggested by Kenton, still
avoids "dangling else" warnings and also allows to have diagnostics on
missing ";" at the end. So this way is better.

c++/src/kj/debug.h

@@ -131,9 +131,10 @@ namespace kj {
 #define KJ_EXPAND(X) X
 
 #define KJ_LOG(severity, ...) \
-{ if (!::kj::_::Debug::shouldLog(::kj::LogSeverity::severity)) {} else \
+  for (bool _kj_shouldLog = ::kj::_::Debug::shouldLog(::kj::LogSeverity::severity); \
+       _kj_shouldLog; _kj_shouldLog = false) \
     ::kj::_::Debug::log(__FILE__, __LINE__, ::kj::LogSeverity::severity, \
-                        "" #__VA_ARGS__, __VA_ARGS__); }
+                        "" #__VA_ARGS__, __VA_ARGS__)
 
 #define KJ_DBG(...) KJ_EXPAND(KJ_LOG(DBG, __VA_ARGS__))
 
@@ -210,9 +211,10 @@ namespace kj {
 #else
 
 #define KJ_LOG(severity, ...) \
-{ if (!::kj::_::Debug::shouldLog(::kj::LogSeverity::severity)) {} else \
+  for (bool _kj_shouldLog = ::kj::_::Debug::shouldLog(::kj::LogSeverity::severity); \
+       _kj_shouldLog; _kj_shouldLog = false) \
     ::kj::_::Debug::log(__FILE__, __LINE__, ::kj::LogSeverity::severity, \
-                        #__VA_ARGS__, ##__VA_ARGS__); }
+                        #__VA_ARGS__, ##__VA_ARGS__)
 
 #define KJ_DBG(...) KJ_LOG(DBG, ##__VA_ARGS__)