[libc++] Move __libcpp_timespec_t into namespace std (#80004)

It was previously defined outside of namespace std for apparently no
good reason.

Author: ldionne

libcxx/include/__thread/support.h

@@ -23,6 +23,8 @@
 // The following functionality must be provided by any implementation:
 //
 
+_LIBCPP_BEGIN_NAMESPACE_STD
+
 using __libcpp_timespec_t = ...;
 
 //
@@ -98,6 +100,8 @@ int __libcpp_tls_create(__libcpp_tls_key*, void (*__at_exit)(void*));
 void* __libcpp_tls_get(__libcpp_tls_key);
 int __libcpp_tls_set(__libcpp_tls_key, void*);
 
+_LIBCPP_END_NAMESPACE_STD
+
 */
 
 #if !defined(_LIBCPP_HAS_NO_THREADS)

libcxx/include/__thread/support/c11.h

@@ -20,10 +20,10 @@
 #  pragma GCC system_header
 #endif
 
-typedef ::timespec __libcpp_timespec_t;
-
 _LIBCPP_BEGIN_NAMESPACE_STD
 
+using __libcpp_timespec_t = ::timespec;
+
 //
 // Mutex
 //

libcxx/include/__thread/support/pthread.h

@@ -36,10 +36,10 @@
 #  pragma GCC system_header
 #endif
 
-typedef ::timespec __libcpp_timespec_t;
-
 _LIBCPP_BEGIN_NAMESPACE_STD
 
+using __libcpp_timespec_t = ::timespec;
+
 //
 // Mutex
 //

libcxx/include/__thread/support/windows.h

@@ -18,10 +18,10 @@
 #  pragma GCC system_header
 #endif
 
-typedef ::timespec __libcpp_timespec_t;
-
 _LIBCPP_BEGIN_NAMESPACE_STD
 
+using __libcpp_timespec_t = ::timespec;
+
 //
 // Mutex
 //