Make clang_compilation_database compatible with variants

In the current implementation if a project is using
build variants it's not possible to use the clang_compilation_database
plugin because it strips the variant information from the build object.

waflib/extras/clang_compilation_database.py

@@ -126,7 +126,7 @@ def patch_execute():
 		Invoke clangdb command before build
 		"""
 		if self.cmd.startswith('build'):
-			Scripting.run_command('clangdb')
+			Scripting.run_command(self.cmd.replace('build','clangdb'))
 
 		old_execute_build(self)