Driver options taking a value typically use `=` as the separator, instead of a
space. Not using a space also helps a dozen compiler driver wrappers which don'tUnfortunately many older driver options do not stick with the rule, but I
want to know which options are `Separate`. Unfortunately many older driverfind -Xclang used a lot and will be convenient if -Xclang= exists.
options do not stick with the rule, but I find -Xclang used a lot and will
be convenient if -Xclang= exists.
For build systems using a string array instead of a string to indicate compiler options,
`["-Xclang=-foo"]` is more convenient than `["-Xclang", "-foo"]`.
If a tool wants to filter out -Xclang=-foo, it is trivial for the `=` form, but
complex for the space separated form.
Note: there is a `=` precedence for `-X*` option: `-Xopenmp-target=`.