Driver options taking a value typically use = as the separator, instead of a
space. Unfortunately many older driver 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.