[llvm-cov] Warn when -arch spec is missing/invalid for universal binary (reland)

llvm-cov reports a poor error message when the -arch specifier is
missing or invalid, and a binary has multiple slices. Make the error
message more specific.

(This version of the patch avoids using llvm::none_of -- the way I used
the utility caused compile errors on many bots, possibly because the
wrong overload of `none_of` was selected.)

rdar://40312677