[AST/index] Introduce an option 'SuppressTemplateArgsInCXXConstructors' in printing policy.

Enable it for USRs and names when indexing.
Forward references can have different template argument names; including them
makes USRs and names unstable, since the name depends on whether we saw a forward reference or not.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@260866 91177308-0d34-0410-b5e6-96231b3b80d8