[XRay] [compiler-rt] fix heap overflow by computing record pointers correctly

Summary:
While there, unify InMemoryRawLog and InMemoryRawLogWithArg's coding style:
- swap libc's memcpy(3) for sanitizer's internal memcpy
- use basic pointer arithmetics to compute offsets from the first record
  entry in the pre-allocated buffer, which is always the appropriate type
  for the given function
- lose the local variable references as the TLD.* names fit just as well

Reviewers: eizan, kpw, dberris, dblaikie

Subscribers: #sanitizers, llvm-commits

Differential Revision: https://reviews.llvm.org/D42289

llvm-svn: 322941