swift234

StdlibUnittest: make testing return-autoreleased optimization reliable

On some platforms (for example, x86_64), the first call to
`objc_autoreleaseReturnValue` will always autorelease because it would
fail to verify the instruction sequence in the caller.  On x86_64
certain PLT entries would be still pointing to the resolver function,
and sniffing the call sequence would fail.

This change adds a "warmup" return-autoreleased sequence to the test
harness.

rdar://18385128

Swift SVN r22127