This is more an RFC at this point. I'm curious about feedback!
One nice feature of the os_signpost API is that format string substitutions happen in the consumer, not the logging application. LLVM's current Signpost class doesn't take advantage of this though and instead always uses a static "Begin/End %s" format string.
This patch is proof of concept for how to use variadic macros to allow the API to be used as intended. Unfortunately, the primary use-case I had in mind (the LLDB_SCOPED_TIMER() macro) does not get much better from this, because __PRETTY_FUNCTION__ is *not* a macro, but a static string, so signposts created by LLDB_SCOPED_TIMER() still use a static "%s" format string. At least LLDB_SCOPED_TIMERF() works as intended.