Teach the type-id/expression disambiguator about different

disambiguation contexts, so that we properly parse template arguments
such as

  A<int()>

as type-ids rather than as expressions. Since this can be confusing
(especially when the template parameter is a non-type template
parameter), we try to give a friendly error message.

Almost, eliminate a redundant error message (that should have been a
note) and add some ultra-basic checks for non-type template
arguments.

llvm-svn: 64189