Use magic statics for non-trivial thread-local globals

This avoids calling the global's constructor on threads that will
never interact with them. Calling the constructor can have surprising
overhead, as e.g. MSVC's Debug mode `std::vector` will allocate in
the default constructor.

Closes #3050

src/catch2/internal/catch_message_info.cpp

@@ -7,20 +7,24 @@
 // SPDX-License-Identifier: BSL-1.0
 
 #include <catch2/internal/catch_message_info.hpp>
+#include <catch2/internal/catch_thread_local.hpp>
 
 namespace Catch {
 
+    namespace {
+        // Messages are owned by their individual threads, so the counter should
+        // be thread-local as well. Alternative consideration: atomic counter,
+        // so threads don't share IDs and things are easier to debug.
+        static CATCH_INTERNAL_THREAD_LOCAL unsigned int messageIDCounter = 0;
+    }
+
     MessageInfo::MessageInfo( StringRef _macroName,
                               SourceLineInfo const& _lineInfo,
                               ResultWas::OfType _type )
     :   macroName( _macroName ),
         lineInfo( _lineInfo ),
         type( _type ),
-        sequence( ++globalCount )
+        sequence( ++messageIDCounter )
     {}
 
-    // Messages are owned by their individual threads, so the counter should be thread-local as well.
-    // Alternative consideration: atomic, so threads don't share IDs and things are easier to debug.
-    CATCH_INTERNAL_THREAD_LOCAL unsigned int MessageInfo::globalCount = 0;
-
 } // end namespace Catch