Remove unneeded forward declarations.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@186244 91177308-0d34-0410-b5e6-96231b3b80d8
Attempt at fixing a mingw bot.

It is failing with

YAMLTest.cpp:38:   instantiated from here
YAMLTraits.h:226: error: 'llvm::yaml::MappingTraits<<unnamed>::BinaryHolder>::mapping' is not a valid template argument for type 'void (*)(llvm::yaml::IO&, <unnamed>::BinaryHolder&)' because function 'static void llvm::yaml::MappingTraits<<unnamed>::BinaryHolder>::mapping(llvm::yaml::IO&, <unnamed>::BinaryHolder&)' has not external linkage

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@186245 91177308-0d34-0410-b5e6-96231b3b80d8