[c++20] P0780R2: Support pack-expansion of init-captures.

This permits an init-capture to introduce a new pack:

  template<typename ...T> auto x = [...a = T()] { /* a is a pack */ };

To support this, the mechanism for allowing ParmVarDecls to be packs has
been extended to support arbitrary local VarDecls.

llvm-svn: 361300