Add clang11

As far as I understand this should allow building with clang11? I will trigger a pipeline Can someone confirm that the commented variables are really not used?

cmake/toolchain/heptools-dev4python2.cmake

@@ -9,7 +9,7 @@ include(heptools-dev-generators)
 LCG_external_package(ROOT    v6.22.06 )
 
 # Remove COOL/CORAL for time being (tests, at least, failing)
-if(${LCG_COMP} MATCHES clang10)
+if(${LCG_COMP} MATCHES clang)
   LCG_remove_package(COOL)
   LCG_remove_package(CORAL)
 endif()