tsan: fix compiler warning

tsan_debugging.cc: In function ‘void* __tsan_get_current_report()’:
tsan_debugging.cc:61:18: warning: cast from type ‘const __tsan::ReportDesc*’
                         to type ‘void*’ casts away qualifiers [-Wcast-qual]
   return (void *)rep;

llvm-svn: 267679