Fix PR18498: Support explicit template arguments with variadic generic lambdas

http://llvm.org/bugs/show_bug.cgi?id=18498

This code was resulting in a crash:

auto L = [](auto ... v) { };
L.operator()<int>(3);

The reason is that the partially-substituted-pack is incorrectly retained within the current-instantiation-scope during template-argument-finalization, and because lambda's are local, there parent instantiation scopes are merged, which leads to the expansion-pattern being retained in the finalized specialization.

This patch ensures that once we have finalized deduction of a parameter-pack, we remove the partially-substituted-pack so that it doesn't cause CheckParameterPacksForExpansion to incorrectly inform the caller that it needs to retain the expansion pattern.


Thanks to Richard Smith for the review!

http://reviews.llvm.org/D2135