Update libcxx skips for a new compiler warning (#5283)

tests/libcxx/expected_results.txt

@@ -1381,3 +1381,9 @@ std/containers/sequences/vector/vector.modifiers/assert.push_back.invalidation.p
 # This test is marked as `REQUIRES: has-unix-headers, libcpp-has-abi-bounded-iterators-in-std-array`.
 # Listing these on separate lines would allow magic_comments.txt to recognize it.
 std/containers/sequences/array/assert.iterators.pass.cpp:9 SKIPPED
+
+# These tests emit C5321, which warns when the resolution to core issue 1656 affects a u8 string literal.
+# The conformant behavior is opt-in because it can silently change behavior.
+# : warning C5321: nonstandard extension used: encoding '\x80' as a multi-byte utf-8 character. Use \u instead for cross platform compatibility or '/Zc:u8EscapeEncoding' to disable the extension.
+std/utilities/format/format.functions/escaped_output.unicode.pass.cpp:9 SKIPPED
+std/utilities/format/format.functions/fill.unicode.pass.cpp:9 SKIPPED