Fix nodiscard failure tests on compilers w/o -verify.

Previously .fail.cpp tests for nodiscard were run with -Wunused-result
being a warning, not an error, when the compiler didn't support -verify.

When -verify isn't enabled this change judiciously adds -Werror=unused-result
when to only the failure tests containing the // expected-error string for nodiscard.

As a drive-by change, this patch also adds a missing // UNSUPPORTED: c++2a to
a test which was only supposed to run in C++ <= 11.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@322776 91177308-0d34-0410-b5e6-96231b3b80d8

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++17
+// UNSUPPORTED: c++14, c++17, c++2a
 // UNSUPPORTED: sanitizer-new-delete
 
 #include <new>

utils/libcxx/test/format.py

@@ -242,7 +242,18 @@ class LibcxxTestFormat(object):
             test_cxx.useWarnings()
             if '-Wuser-defined-warnings' in test_cxx.warning_flags:
                 test_cxx.warning_flags += ['-Wno-error=user-defined-warnings']
-
+        else:
+            # We still need to enable certain warnings on .fail.cpp test when
+            # -verify isn't enabled. Such as -Werror=unused-result. However,
+            # we don't want it enabled too liberally, which might incorrectly
+            # allow unrelated failure tests to 'pass'.
+            #
+            # Therefore, we check if the test was expected to fail because of
+            # nodiscard before enabling it
+            test_str = "ignoring return value of function declared with " \
+              + "'nodiscard' attribute"
+            if test_str in contents:
+                test_cxx.flags += ['-Werror=unused-result']
         cmd, out, err, rc = test_cxx.compile(source_path, out=os.devnull)
         expected_rc = 0 if use_verify else 1
         if rc == expected_rc: