Change test suite to support c++17 dialect flag instead of c++1z.

This patch changes the test suite to attempt and prefer -std=c++17 over
-std=c++1z. It also fixes the REQUIRES and UNSUPPORTED lit markers
to refer to c++17 over c++1z.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@317610 91177308-0d34-0410-b5e6-96231b3b80d8

test/std/iterators/stream.iterators/istream.iterator/istream.iterator.cons/default.pass.cpp

@@ -9,7 +9,7 @@
 
 // Usage of is_trivially_constructible is broken with these compilers.
 // See https://bugs.llvm.org/show_bug.cgi?id=31016
-// XFAIL: clang-3.7, apple-clang-7 && c++1z
+// XFAIL: clang-3.7, apple-clang-7 && c++17
 
 // <iterator>
 

test/std/language.support/support.dynamic/new.delete/new.delete.array/sized_delete_array11.pass.cpp

@@ -12,7 +12,7 @@
 // Note that sized delete operator definitions below are simply ignored
 // when sized deallocation is not supported, e.g., prior to C++14.
 
-// UNSUPPORTED: c++14, c++1z
+// UNSUPPORTED: c++14, c++17
 // UNSUPPORTED: sanitizer-new-delete
 
 #include <new>

test/std/language.support/support.dynamic/new.delete/new.delete.single/sized_delete11.pass.cpp

@@ -12,7 +12,7 @@
 // Note that sized delete operator definitions below are simply ignored
 // when sized deallocation is not supported, e.g., prior to C++14.
 
-// UNSUPPORTED: c++14, c++1z
+// UNSUPPORTED: c++14, c++17
 // UNSUPPORTED: sanitizer-new-delete
 
 #include <new>