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>.
The SILGen testsuite consists of valid Swift code covering most language
features. We use these tests to verify that no unknown nodes are in the
file's libSyntax tree. That way we will (hopefully) catch any future
changes or additions to the language which are not implemented in
libSyntax.
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.
Now that we have interface types in witness tables, make sure to set the
appropriate generic environment when printing witness tables. This
ensures that we get the resugared generic type parameter names.
