SIL functions for AST declarations do this, and the SIL verifier enforces
this, so let's do it for witness thunks too, fixing a devirtualizer
crash.
Fixes <rdar://problem/46571799>.
Conditional conformances aren't quite ready yet for Swift 4.1, so
introduce the flag `-enable-experimental-conditional-conformances` to
enable conditional conformaces, and an error when one declares a
conditional conformance without specifying the flag.
Add this flag when building the standard library (which will vend
conditional conformances) and to all of the tests that need it.
Fixes rdar://problem/35728337.
