meson: use a feature option for microsoft-clc

It's less code and makes the configuration easier to fine tune.

Fixes: ff05da7f8d
       ("microsoft: Add CLC frontend and kernel/compute support to DXIL converter")

Reviewed-by: Jesse Natalie <jenatali@microsoft.com>
Reviewed-by: Eric Engestrom <eric@engestrom.ch>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/7699>

meson_options.txt

@@ -268,9 +268,8 @@ option(
 )
 option(
   'microsoft-clc',
-  type : 'combo',
+  type : 'feature',
   value : 'auto',
-  choices : ['auto', 'true', 'false'],
   description : 'Build support for the Microsoft CLC to DXIL compiler'
 )
 option(