llvm: replace @when with internal check in @run_before (#8092)

Fixes #8088

#7012 added a @when condition for a @run_before check to constrain
that check to only run on Darwin. @when is intended to be used to
choose one of several different implementations of a given function
and cannot be used to conditionally deactivate a check altogether.

This replaces the external decorator with a check that executes at
the beginning of the function.

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

@@ -415,9 +415,10 @@ class Llvm(CMakePackage):
     # Github issue #4986
     patch('llvm_gcc7.patch', when='@4.0.0:4.0.1+lldb %gcc@7.0:')
 
-    @when('+lldb platform=darwin')
     @run_before('cmake')
     def check_darwin_lldb_codesign_requirement(self):
+        if not self.spec.satisfies('+lldb platform=darwin'):
+            return
         codesign = which('codesign')
         cp = which('cp')
         mkdir('tmp')