Avoid UB in upb by switching to the XCT section, which will run our constructors before the compiler-generated initializers.

#test-continuous

This is detailed more in https://devblogs.microsoft.com/cppblog/new-compiler-warnings-for-dynamic-initialization/.

PiperOrigin-RevId: 920402652

upb/port/def.inc

diff --git a/upb/port/def.inc b/upb/port/def.inc
index 4616c65..b398a95 100644
--- a/upb/port/def.inc
+++ b/upb/port/def.inc
@@ -668,10 +668,10 @@
  * to prevent optimizers from removing the constructor. Our solution is to
  * create a dummy exported weak symbol that prevent this stripping.
  */
-#pragma section(".CRT$XCU", long, read)
+#pragma section(".CRT$XCT", read)
 #define UPB_CONSTRUCTOR(name, unique_name)                                   \
   static void __cdecl UPB_PRIVATE(name)(void);                               \
-  __declspec(allocate(".CRT$XCU"), selectany) void(                          \
+  __declspec(allocate(".CRT$XCT"), selectany) void(                          \
       __cdecl * UPB_PRIVATE(name##_))(void) = UPB_PRIVATE(name);             \
   __declspec(selectany, dllexport) void* UPB_PRIVATE(name##_force_linkage) = \
       &UPB_PRIVATE(name##_);                                                 \