Move the libstdc++ code path to a separate file.

We need to #include <exception> for this to work, but this brings in the
public definition of `std::type_info` and, unfortunately, that breaks
the private definition that we need in this compilation unit.

CMakeLists.txt

@@ -82,7 +82,7 @@ if (WIN32)
 	list(APPEND libobjc_CXX_SRCS eh_win32_msvc.cc)
 else ()
 	list(APPEND libobjc_C_SRCS eh_personality.c)
-	set(libobjcxx_CXX_SRCS objcxx_eh.cc)
+	set(libobjcxx_CXX_SRCS objcxx_eh.cc libstdcxx_current_primary_exception.cc)
 endif (WIN32)
 
 

libstdcxx_current_primary_exception.cc

@@ -0,0 +1,19 @@
+#include "visibility.h"
+#include <exception>
+
+#ifdef __GLIBCXX__
+/**
+ * libsupc++ doesn't expose __cxa_current_primary_exception, so implement this
+ * using the libstdc++ wrapper.  The exception pointer in the
+ * `std::exception_ptr` object is reference counted, so stealing it by poking
+ * at the pointer directly means that we acquire it with a reference count of
+ * 1.
+ */
+PRIVATE extern "C" void *__cxa_current_primary_exception()
+{
+	std::exception_ptr p = std::current_exception();
+	void *obj = *(void**)&p;
+	*(void**)&p = nullptr;
+	return obj;
+}
+#endif

objcxx_eh.cc

@@ -22,19 +22,8 @@ extern "C"
 void __cxa_throw(void *thrown_exception, std::type_info *tinfo,
                  void (*dest)(void *));
 
-#ifdef __GLIBCXX__
-#include <exception>
-static void *__cxa_current_primary_exception()
-{
-	std::exception_ptr p = std::current_exception();
-	void *obj = *(void**)&p;
-	*(void**)&p = nullptr;
-	return obj;
-}
-#else
 extern "C"
 void *__cxa_current_primary_exception();
-#endif
 
 using namespace __cxxabiv1;