[XRay] [compiler-rt] Write buffer length to FDR log before writing buffer.

Summary:
Currently the FDR log writer, upon flushing, dumps a sequence of buffers from
its freelist to disk. A reader can read the first buffer up to an EOB record,
but then it is unclear how far ahead to scan to find the next threads traces.

There are a few ways to handle this problem.
1. The reader has externalized knowledge of the buffer size.
2. The size of buffers is in the file header or otherwise encoded in the log.
3. Only write out the portion of the buffer with records. When released, the
   buffers are marked with a size.
4. The reader looks for memory that matches a pattern and synchronizes on it.

2 and 3 seem the most flexible and 2 does not rule 3 out.

This is an implementation of 2.

In addition, the function handler for fdr more aggressively checks for
finalization and makes an attempt to release its buffer.

Reviewers: pelikan, dberris

Reviewed By: dberris

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D31384

llvm-svn: 298982