[XRay][compiler-rt] FDR Mode: Add extents metadata to buffer

When providing raw access to the FDR mode buffers, we used to not
include the extents metadata record. This oversight means that
processing the buffers in-memory will lose important information that
would have been written in files.

This change exposes the metadata record by serializing the data
similarly to how we would do it when flushing to files.

llvm-svn: 337441