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;
                                            ^

clang/include/clang/Analysis/Analyses/Dominators.h

@@ -108,15 +108,16 @@ class CfgTraits : public llvm::CfgTraits<CfgTraitsBase, CfgTraits> {
 
 } // namespace clang
 
-template <> struct llvm::CfgTraitsFor<clang::CFGBlock> {
+namespace llvm {
+
+template <> struct CfgTraitsFor<clang::CFGBlock> {
   using CfgTraits = clang::CfgTraits;
 };
 
 // FIXME: There is no good reason for the domtree to require a print method
 // which accepts an LLVM Module, so remove this (and the method's argument that
 // needs it) when that is fixed.
 
-namespace llvm {
 
 class Module;