Implement code completion within a function call, triggered after the

opening parentheses and after each comma. We gather the set of visible
overloaded functions, perform "partial" overloading based on the set
of arguments that we have thus far, and return the still-viable
results sorted by the likelihood that they will be the best candidate.

Most of the changes in this patch are a refactoring of the overloading
routines for a function call, since we needed to separate out the
notion of building an overload set (common to code-completion and
normal semantic analysis) and then what to do with that overload
set. As part of this change, I've pushed explicit template arguments
into a few more subroutines.

There is still much more work to do in this area. Function templates
won't be handled well (unless we happen to deduce all of the template
arguments before we hit the completion point), nor will overloaded
function-call operators or calls to member functions.

llvm-svn: 82549