profile: Avoid name collisions between instrumentation and runtime

The naming scheme we're using for counters in profile data shares a
prefix with some fixed names we use for the runtime, notably
__llvm_profile_data_begin and _end. Embarrassingly, this means a
function called begin() can't be instrumented.

This modifies the runtime names so as not to collide with the
instrumentation.

llvm-svn: 217166