ignore missing python bindings in llvm 17+

The python bindings to llvm have been removed in version 17, but those
in the clang portion of the project still exist.  This ignores the now
missing llvm bindings but retains the variant to control installing the
clang bindings.

var/spack/repos/builtin/packages/llvm/package.py

@@ -977,6 +977,8 @@ def post_install(self):
                 ninja()
                 ninja("install")
         if "+python" in self.spec:
+            if spec.version < Version("17.0.0"):
+                # llvm bindings were removed in v17: https://releases.llvm.org/17.0.1/docs/ReleaseNotes.html#changes-to-the-python-bindings
                 install_tree("llvm/bindings/python", python_platlib)
 
             if "+clang" in self.spec: