PR42587: diagnose unexpanded uses of a pack parameter of a generic

PR42587: diagnose unexpanded uses of a pack parameter of a generic
lambda from within the lambda-declarator.

Instead of trying to reconstruct whether a parameter pack was declared
inside a lambda (which we can't do correctly in general because we might
not have attached parameters to their declaration contexts yet), track
the set of parameter packs introduced in each live lambda scope, and
require only those parameters to be immediately expanded when they
appear inside that lambda.

In passing, fix incorrect disambiguation of a lambda-expression starting
with an init-capture pack in a braced-init-list. We previously
incorrectly parsed that as a designated initializer.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@369985 91177308-0d34-0410-b5e6-96231b3b80d8