[GWP-ASan] Update backtrace function signature.

Summary:
Updates the function signature and comments for backtracing (and printing
backtraces). This update brings GWP-ASan in line with future requirements for
stack frame compression, wherein the length of the trace is provided
explicitly, rather than relying on nullptr-termination.

Reviewers: vlad.tsyrklevich

Reviewed By: vlad.tsyrklevich

Subscribers: #sanitizers, llvm-commits, morehouse

Tags: #sanitizers, #llvm

Differential Revision: https://reviews.llvm.org/D66099

llvm-svn: 368619