[ModuleInterface] Allow conformances to be missing value witnesses (#18932)

It's not clear whether we'll actually need this feature in the long
run, but we certainly need it now because non-@usableFromInline
members can (currently) satisfy public requirements when a
@usableFromInline internal type conforms to a public protocol. In
these cases, we'll treat the witnesses as present but opaque, and
clients will perform dynamic dispatch when using them even when
a generic function gets specialized.

With this, we're able to generate a textual interface for the standard
library, compile it back to a swiftmodule, and use it to build a Hello
World program!

lib/Serialization/Deserialization.cpp

@@ -2697,6 +2697,7 @@ ModuleFile::getDeclCheckedImpl(DeclID DID) {
       return nullptr;
     }
 
+    theStruct->setAddedImplicitInitializers();
     if (isImplicit)
       theStruct->setImplicit();
     theStruct->setIsObjC(isObjC);
@@ -3604,6 +3605,7 @@ ModuleFile::getDeclCheckedImpl(DeclID DID) {
       return nullptr;
     }
 
+    theEnum->setAddedImplicitInitializers();
     if (isImplicit)
       theEnum->setImplicit();
     theEnum->setIsObjC(isObjC);