Metal: Avoid leaking MTLFunctionConstantValues in ProgramMtl Hold with ObjCObj<>. Patch by David Kilzer <ddkilzer@apple.com> Bug: angleproject:6920 Change-Id: I635097100b1e004e9276490ff8574870e1c5f33f Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/3568379 Reviewed-by: Jonah Ryan-Davis <jonahr@google.com> Reviewed-by: Kenneth Russell <kbr@chromium.org> Commit-Queue: Kenneth Russell <kbr@chromium.org>