Implement the part of C++0x [temp.arg.explicit]p3 that pertains to

parameter packs. In particular, a parameter pack not otherwise deduced
is deduced to an empty parameter pack.

The C++0x wording here is a bit unfortunate; this should really only
apply to function templates, and it mentions "trailing" parameter
packs, which doesn't really make sense in the context of function
templates. Will file a core issue separately.

llvm-svn: 122463