Revert r195526, "Remove uninstantiable constructor from DeclTemplate.h"

It apparently causes some versions of GCC to segfault.
http://bb.pgr.jp/builders/cmake-clang-x86_64-linux/builds/17175/steps/build_clang/logs/stdio

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@195529 91177308-0d34-0410-b5e6-96231b3b80d8