Initial support for C++ parameter completion

The improved completion in call context now works with:

 - Functions.
 - Member functions.
 - Constructors.
 - New expressions.
 - Function call expressions.
 - Template variants of the previous.

There are still rough edges to be fixed:

 - Provide support for optional parameters.         (fix known)
 - Provide support for member initializers.         (fix known)
 - Provide support for variadic template functions. (fix unknown)
 - Others?

llvm-svn: 226670