[XRay] Guard local variables with `static` and struct with unnamed namespaces

Summary:
This is for coding standard conformance, and for fixing an ODR violation
issue: __xray::ThreadLocalData is defined twice and differently in
xray_fdr_logging.cc and xray_basic_logging.cc

Reviewers: dberris, mboerger, eizan

Reviewed By: dberris

Subscribers: delcypher, jfb, llvm-commits, #sanitizers

Differential Revision: https://reviews.llvm.org/D52639

llvm-svn: 343289