This takes care of the standard library portion, but we need a new
BuiltinUTF16ExtendedGraphemeClusterLiteralConvertible protocol in order to
fully recover the performance of character literals.
Note that part of the character_literals.swift test is currently disabled. That
will need to be fixed before we can merge this work.
This is done by ensuring that the corresponding Character constructor is inlined. llvm will do the constant folding.
Also add a test which checks this.
It makes character literals much faster (3x improvement for the CharacterLiteralsSmall benchmark)
And it removes _a lot_ of redundant code (~80% for the CharacterLiteralsSmall benchmark)
