[test] Skip a @differentiable test during back deployment testing

This currently fails when run on the macOS 10.14.4 stdlib:

```
[ RUN      ] FunctionTypeMetadata.Reflect differentiable function type
stdout>>> check failed at /Users/buildslave/jenkins/workspace/fs-swift-main-tools_RA-stdlib_RA-macOS-swift-in-OS-10.14.4/swift/test/AutoDiff/validation-test/function_type_metadata.swift, line 10
stdout>>> expected: "@differentiable(reverse) (Swift.Float) -> Swift.Float" (of type Swift.String)
stdout>>> actual: "-3.8442426e-13" (of type Swift.String)
stderr>>> CRASHED: SIGSEGV
the test crashed unexpectedly
[     FAIL ] FunctionTypeMetadata.Reflect differentiable function type
```

rdar://76039256

Author: lorentey

test/AutoDiff/validation-test/function_type_metadata.swift

@@ -1,6 +1,9 @@
 // RUN: %target-run-simple-swift
 // REQUIRES: executable_test
 
+// rdar://76039256
+// UNSUPPORTED: use_os_stdlib
+
 import StdlibUnittest
 import _Differentiation