meson: use a feature option for llvm

Still not as awesome as it should be, but an improvement over what we had
before.

Reviewed-by: Jesse Natalie <jenatali@microsoft.com>
Acked-by: Alyssa Rosenzweig <alyssa.rosenzweig@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/20409>

meson.build

@@ -1681,17 +1681,10 @@ else
   _shared_llvm = (_shared_llvm == 'enabled')
 endif
 _llvm = get_option('llvm')
-if _llvm == 'true'
-  _llvm = 'enabled'
-  warning('llvm option "true" deprecated, please use "enabled" instead.')
-elif _llvm == 'false'
-  _llvm = 'disabled'
-  warning('llvm option "false" deprecated, please use "disabled" instead.')
-endif
 
 dep_llvm = null_dep
 with_llvm = false
-if _llvm != 'disabled'
+if _llvm.allowed()
   dep_llvm = dependency(
     'llvm',
     version : _llvm_version,
@@ -1699,7 +1692,7 @@ if _llvm != 'disabled'
     optional_modules : llvm_optional_modules,
     required : (
       with_amd_vk or with_gallium_radeonsi or with_gallium_opencl or with_clc
-      or _llvm == 'enabled'
+      or _llvm.enabled()
     ),
     static : not _shared_llvm,
     fallback : ['llvm', 'dep_llvm'],

meson_options.txt

@@ -317,9 +317,7 @@ option(
 )
 option(
   'llvm',
-  type : 'combo',
-  value : 'auto',
-  choices : ['auto', 'true', 'false', 'enabled', 'disabled'],
+  type : 'feature',
   description : 'Build with LLVM support.'
 )
 option(