Fix docs generation for the AST matchers:

1. Move internal functions into ASTMatchersInternal.
2. Adapt dump_ast_matchers.py to the new VariadicOperatorMatcherFunc
   signature.
3. Update the actual docs with the updated tool / code.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@202017 91177308-0d34-0410-b5e6-96231b3b80d8