[lldb] Ad os_signpost support to lldb_private::Timer

Emit os_signposts when supported from LLDB's timer class. A vast amount
of performance sensitive places in LLDB are already instrumented with
the Timer class.

By emitting signposts we can examine this information in Instruments. I
recommend looking at Daniel's differential for why this is so powerful:
https://reviews.llvm.org/D52954.

Differential revision: https://reviews.llvm.org/D93657