libstdc++: Break header cycle between <new> and <exception>

The <new> and <exception> headers each include each other, which makes
building them as header-units "exciting". The <new> header only needs
the definition of std::exception (in order to derive from it) which is
already in its own header, so just include that.

libstdc++-v3/ChangeLog:

	* include/bits/stl_iterator.h: Include <bits/exception_defines.h>
	for definitions of __try, __catch and __throw_exception_again.
	(counted_iterator::operator++(int)): Use __throw_exception_again
	instead of throw.
	* libsupc++/new: Include <bits/exception.h> not <exception>.
	* libsupc++/new_opvnt.cc: Include <bits/exception_defines.h>.
	* testsuite/18_support/destroying_delete.cc: Include
	<type_traits> for std::is_same_v definition.
	* testsuite/20_util/variant/index_type.cc: Qualify size_t.

libstdc++-v3/include/bits/stl_iterator.h

@@ -79,6 +79,7 @@
 #if __cplusplus > 201703L
 # include <compare>
 # include <new>
+# include <bits/exception_defines.h>
 # include <bits/iterator_concepts.h>
 #endif
 
@@ -2062,7 +2063,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 	    return _M_current++;
 	  } __catch(...) {
 	    ++_M_length;
-	    throw;
+	    __throw_exception_again;
 	  }
 
       }

libstdc++-v3/libsupc++/new

@@ -38,7 +38,7 @@
 #pragma GCC system_header
 
 #include <bits/c++config.h>
-#include <exception>
+#include <bits/exception.h>
 
 #pragma GCC visibility push(default)
 
@@ -52,7 +52,7 @@ namespace std
    *
    *  @c bad_alloc (or classes derived from it) is used to report allocation
    *  errors from the throwing forms of @c new.  */
-  class bad_alloc : public exception 
+  class bad_alloc : public exception
   {
   public:
     bad_alloc() throw() { }

libstdc++-v3/libsupc++/new_opvnt.cc

@@ -25,7 +25,8 @@
 
 #include <bits/c++config.h>
 #include "new"
- 
+#include "exception_defines.h"
+
 _GLIBCXX_WEAK_DEFINITION void*
 operator new[] (std::size_t sz, const std::nothrow_t&) noexcept
 {

libstdc++-v3/testsuite/18_support/destroying_delete.cc

@@ -19,6 +19,7 @@
 // { dg-do run { target c++2a } }
 
 #include <new>
+#include <type_traits>
 #include <testsuite_hooks.h>
 
 #ifndef __cpp_lib_destroying_delete

libstdc++-v3/testsuite/20_util/variant/index_type.cc

@@ -22,4 +22,4 @@
 #include <variant>
 
 static_assert(sizeof(std::variant<signed char, unsigned char>)
-	      < sizeof(size_t));
+	      < sizeof(std::size_t));