[XRay] [compiler-rt] Simplify FDR logging handler. [NFC]

Summary:
Not repeating screamy failure paths makes the 300+ line function a bit shorter.
There's no need to overload the variable name "Buffer" if it only works on the
thread local buffer.  Fix some comments while there.

I plan to move the rewinding logic into a separate function too, but in this
diff it would be too much of a mess to comprehend.  This is trivially NFC.

Reviewers: kpw, dberris

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D31930

llvm-svn: 300018