libstdc++: Remove redundant noexcept-specifier on definitions

These destructors are noexcept anyway. I removed the redundant noexcept
from the error_category destructor's declaration in r0-123475, but
didn't remove it from the defaulted definition in system_error.cc. That
causes warnings if the library is built with Clang.

This removes the redundant noexcept from ~error_category and
~system_error and adds tests to ensure they really are noexcept.

Signed-off-by: Jonathan Wakely <jwakely@redhat.com>

libstdc++-v3/ChangeLog:

	* src/c++11/system_error.cc (error_category::~error_category()):
	Remove noexcept-specifier.
	(system_error::~system_error()): Likewise.
	* testsuite/19_diagnostics/error_category/noexcept.cc: New test.
	* testsuite/19_diagnostics/system_error/noexcept.cc: New test.

libstdc++-v3/src/c++11/system_error.cc

@@ -338,7 +338,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
     _GLIBCXX_THROW_OR_ABORT(system_error(error_code(__i, generic_category())));
   }
 
-  error_category::~error_category() noexcept = default;
+  error_category::~error_category() = default;
 
   const error_category&
   _V2::system_category() noexcept { return system_category_instance; }
@@ -346,7 +346,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
   const error_category&
   _V2::generic_category() noexcept { return generic_category_instance; }
 
-  system_error::~system_error() noexcept = default;
+  system_error::~system_error() = default;
 
   error_condition
   error_category::default_error_condition(int __i) const noexcept

libstdc++-v3/testsuite/19_diagnostics/error_category/noexcept.cc

+// { dg-do compile { target c++11 } }
+#include <system_error>
+
+extern const std::error_category& cat;
+
+static_assert(std::is_nothrow_destructible<std::error_category>::value, "");
+static_assert(noexcept(cat.name()), "");
+static_assert(noexcept(cat.default_error_condition(1)), "");
+static_assert(noexcept(cat.equivalent(1, {})), "");
+static_assert(noexcept(cat.equivalent({}, 1)), "");
+static_assert(noexcept(cat == cat), "");
+static_assert(noexcept(cat != cat), "");
+static_assert(noexcept(cat < cat), "");

libstdc++-v3/testsuite/19_diagnostics/system_error/noexcept.cc

+// { dg-do compile { target c++11 } }
+#include <system_error>
+
+static_assert(std::is_nothrow_destructible<std::system_error>::value, "");
+static_assert(noexcept(std::declval<const std::system_error&>().code()), "");
+static_assert(noexcept(std::declval<const std::system_error&>().what()), "");