libstdc++: Fix ODR violations caused by <tr1/functional>

libstdc++: Fix ODR violations caused by <tr1/functional>

The placeholders for std::tr1::bind are defined in an anonymous
namespace, which means they have internal linkage. This will cause ODR
violations when used in function templates (such as std::tr1::bind) from
multiple translation units. Although probably harmless (every definition
will generate identical code, even if technically ill-formed) we can
avoid the ODR violations by reusing the std::placeholder objects as the
std::tr1::placeholder objects.

To make this work, the std::_Placeholder type needs to be defined for
C++98 mode, so that <tr1/functional> can use it. The members of the
std::placeholder namespace must not be defined by <functional> in C++98
mode, because "placeholders", "_1", "_2" etc. are not reserved names in
C++98. Instead they can be declared in <tr1/functional>, because those
names *are* reserved in that header. With the std::placeholders objects
declared, a simple using-directive suffices to redeclare them in
namespace std::tr1::placeholders. This means any use of the TR1
placeholders actually refers to the C++11 placeholders, which are
defined with external linkage and exported from the library, so don't
cause ODR violations.

libstdc++-v3/ChangeLog:

	* include/std/functional (std::_Placeholder): Define for C++98
	as well as later standards.
	* include/tr1/functional (std::placeholders::_1 etc): Declare
	for C++98.
	(tr1::_Placeholder): Replace with using-declaration for
	std::_Placeholder.
	(tr1::placeholders::_1 etc.): Replace with using-directive for
	std::placeholders.