Try to make GCC5 happy about the CfgTraits thing

It was failing with:

In file included from /work/llvm.monorepo/clang/lib/Analysis/Dominators.cpp:9:0:
/work/llvm.monorepo/clang/include/clang/Analysis/Analyses/Dominators.h: At global scope:
/work/llvm.monorepo/clang/include/clang/Analysis/Analyses/Dominators.h:111:26:
error: specialization of ‘template<class CfgRelatedTypeT> struct llvm::CfgTraitsFor’ in different namespace [-fpermissive]
 template <> struct llvm::CfgTraitsFor<clang::CFGBlock> {
                          ^
In file included from /work/llvm.monorepo/clang/include/clang/Analysis/Analyses/Dominators.h:21:0,
                 from /work/llvm.monorepo/clang/lib/Analysis/Dominators.cpp:9:
/work/llvm.monorepo/llvm/include/llvm/Support/CfgTraits.h:294:44:
error:   from definition of ‘template<class CfgRelatedTypeT> struct llvm::CfgTraitsFor’ [-fpermissive]
 template <typename CfgRelatedTypeT> struct CfgTraitsFor;
                                            ^