[new AA] implement aaLiteral

This functions will be used to constructing AA values: literals and nulls. When this PR will be merged, I will create PR for dmd, which uses this functions, and when dmd PR will be merged, I'll create PR, which rewrite this functions to use new AA implementation.
auto ref causes an unexpected troubles on some tests. May be it's a compiler bug, may be it is my bug in dmd code. If D-Programming-Language/dmd#3904 will be reviewed by someone dmd guru like @9rnsr my possible bug can be found. But for review 3904 we need merge this code.
I think this is not big issue, because this code will be replaced with new AA fast enough (I hope until the next release).