PR13470: Ensure that copy-list-initialization isntantiates as

PR13470: Ensure that copy-list-initialization isntantiates as
copy-list-initialization (and doesn't add an additional copy step):

Fill in the ListInitialization bit when creating a CXXConstructExpr. Use it
when instantiating initializers in order to correctly handle instantiation of
copy-list-initialization. Teach TreeTransform that function arguments are
initializations, and so need this special treatment too. Finally, remove some
hacks which were working around SubstInitializer's shortcomings.

llvm-svn: 170489