[XRay][tools] Rename llvm-xray filenames from .cc -> .cpp (NFC)

Summary:
This brings the filenames in accordance to the style guide and LLVM
conventions for C++ filenames.

As suggested by rnk@ in D46068.

Reviewers: rnk

Subscribers: mgorny, mgrang, llvm-commits

Differential Revision: https://reviews.llvm.org/D46301

llvm-svn: 331321