gnu: llvm-for-mesa: Update to llvm-18.

* gnu/packages/llvm.scm (llvm-for-mesa): Update to llvm-18.
* gnu/packages/gl.scm (mesa)[inputs]: Always use llvm-for-mesa (remove llvm-18
for aarch64 to use llvm-for-mesa).
(mesa-opencl)[native-inputs]: Remove clang-15.  Add clang-18.

Change-Id: I767d40d62a827864d3d109c44f82a43302875eb8

gnu/packages/gl.scm

@@ -324,15 +324,15 @@ also known as DXTn or DXTC) for Mesa.")
     (inputs
      (append
        (if (target-aarch64?)
-           (list clang-18
-                 llvm-18)
-           (list llvm-for-mesa))
+           (list clang-18)
+           '())
        (list elfutils                   ;libelf required for r600 when using llvm
              expat
              (force libva-without-mesa)
              libxml2
              libxrandr
              libxvmc
+             llvm-for-mesa
              vulkan-loader
              wayland
              wayland-protocols
@@ -603,7 +603,7 @@ from software emulation to complete hardware acceleration for modern GPUs.")
      (if (target-aarch64?)
          (package-native-inputs mesa)
          (modify-inputs (package-native-inputs mesa)
-           (prepend clang-15))))))
+           (prepend clang-18))))))
 
 (define-public mesa-opencl-icd
   (package/inherit mesa-opencl

gnu/packages/llvm.scm

@@ -2158,7 +2158,7 @@ using @code{clang-rename}.")))
 
 (define-public llvm-for-mesa
   ;; Note: update the 'clang' input of mesa-opencl when bumping this.
-  (let ((base-llvm llvm-15))
+  (let ((base-llvm llvm-18))
     (package
       (inherit base-llvm)
       (name "llvm-for-mesa")