Derive toolchain classes from ClangGcc instead of both Clang and Gcc
Don't set COMPILER_FAMILY in Clang compiler class

Clang does not have Fortran support, and thus it is not a complete compiler as
far as EasyBuild is concerned.
Set COMPILER_FAMILY in ClangGcc because it is a complete compiler, including
Fortran support
Verify that CC and CXX are set correctly