[XRay] fix and clarify comments in the log file decoder

Summary:
For readers unfamiliar with the XRay code base, reference the compiler-rt
implementation even though we're not allowed to share any code and explain
our little-endian views more clearly.

For code clarity either get rid of obvious comments or explain their
intentions, fix typos, correct coding style according to LLVM's standards
and manually CSE long expressions to point out it is the same expression.

Reviewers: dberris

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D34339

llvm-svn: 313340