libstdc++: Fix warning regressions in <bits/stl_tempbuf.h>

libstdc++: Fix warning regressions in <bits/stl_tempbuf.h>

I caused some new warnings with -Wsystem-headers with my recent changes
to std::get_temporary_buffer and std::_Temporary_buffer. There's a
-Wsign-compare warning which can be avoided by casting the ptrdiff_t
argument to size_t (which also conveniently rejects negative values).

There's also a -Wdeprecated-declarations warning because I moved where
std::get_temporary_buffer is called, but didn't move the diagnostic
pragmas that suppress the warning for calling it.

libstdc++-v3/ChangeLog:

	* include/bits/stl_tempbuf.h (__get_temporary_buffer): Cast
	argument to size_t to handle negative values and suppress
	-Wsign-compare warning.
	(_Temporary_buffer): Move diagnostic pragmas to new location of
	call to std::get_temporary_buffer.