[libc++] Fix tests failing with Clang after removing GCC warnings

Author: ldionne

libcxx/src/filesystem/filesystem_common.h

@@ -198,7 +198,7 @@ struct ErrorHandler {
 using chrono::duration;
 using chrono::duration_cast;
 
-using TimeSpec = std::timespec;
+using TimeSpec = timespec;
 using StatT = struct stat;
 
 template <class FileTimeT, class TimeT,

libcxx/test/std/input.output/filesystems/fs.op.funcs/fs.op.last_write_time/last_write_time.pass.cpp

@@ -33,7 +33,7 @@
 
 using namespace fs;
 
-using TimeSpec = std::timespec;
+using TimeSpec = timespec;
 using StatT = struct stat;
 
 using Sec = std::chrono::duration<file_time_type::rep>;

libcxx/test/std/utilities/meta/meta.trans/meta.trans.other/result_of11.pass.cpp

@@ -19,8 +19,8 @@
 
 // Ignore warnings about volatile in parameters being deprecated.
 // We know it is, but we still have to test it.
-#ifdef __GNUC__
-#pragma GCC diagnostic ignored "-Wvolatile"
+#if defined(__GNUC__) && !defined(__clang__)
+#   pragma GCC diagnostic ignored "-Wvolatile"
 #endif
 
 struct wat