[Driver] Add -Xclang= as an alias for -Xclang

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.

Reviewed By: jhuber6

Differential Revision: https://reviews.llvm.org/D131455