swift234

Provide DynamicMemberRefExpr with .Some(fn)/.None helper expressions.

These helper expressions will eventually be used by SILGen to help
package up the optional values. I expect that we'll eventually have
library builtins for this, so consider this a stop-gap until those
appear.

As part of this, make OpaqueValueExpr a bit more usable: it can now
persist in the AST as a placeholder, but its uses must be within AST
subtrees of some specific introduction point (similarly to how Clang's
OpaqueValueExpr works).


Swift SVN r8051