Initial support for C++ parameter completion

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?

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@226670 91177308-0d34-0410-b5e6-96231b3b80d8