Add partial semantic checking of template arguments that are meant for

non-type template parameters of pointer-to-object and
pointer-to-function type. The most fun part of this is the use of
overload resolution to pick a function from the set of overloaded
functions that comes in as a template argument.

Also, fixed two minor bugs in this area:
  - We were allowing non-type template parameters of type pointer to
  void.
  - We weren't patching up an expression that refers to an overloaded
  function set via "&f" properly.

We're still not performing complete checking of the expression to be
sure that it is referring to an object or function with external
linkage (C++ [temp.arg.nontype]p1).

llvm-svn: 64266