Downgrade the "variadic templates are a C++0x feature" error to an

Downgrade the "variadic templates are a C++0x feature" error to an
ExtWarn. We want variadic templates to be usable in libc++/libstdc++
headers even when we're in C++98/03 mode, since it's the only clean
way to implement TR1 <functional>.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@123852 91177308-0d34-0410-b5e6-96231b3b80d8