[ADT] Adds equality operators for DenseMap and DenseSet, and an initializer_list

[ADT] Adds equality operators for DenseMap and DenseSet, and an initializer_list
constructor for DenseMap (DenseSet already had an initializer_list constructor).

These changes make it easier to migrate existing code that uses std::map and
std::set (which support initializer_list construction and equality comparison)
to DenseMap and DenseSet.

llvm-svn: 344522