[XRay] Rename Buffer.Buffer to Buffer.Data

Summary: some compiler (msvc) treats Buffer.Buffer as constructor and refuse to compile. NFC

Authored by comicfans44.

Reviewers: rnk, dberris

Reviewed By: dberris

Subscribers: llvm-commits

Tags: #sanitizers

Differential Revision: https://reviews.llvm.org/D40346

llvm-svn: 324807