Type check ABI decls

Sema now type-checks the alternate ABI-providing decls inside of @abi attributes.

Making this work—particularly, making redeclaration checking work—required making name lookup aware of ABI decls. Name lookup now evaluates both API-providing and ABI-providing declarations. In most cases, it will filter ABI-only decls out unless a specific flag is passed, in which case it will filter API-only decls out instead. Calls that simply retrieve a list of declarations, like `IterableDeclContext::getMembers()` and friends, typically only return API-providing decls; you have to access the ABI-providing ones through those.

As part of that work, I have also added some basic compiler interfaces for working with the API-providing and ABI-providing variants. `ABIRole` encodes whether a declaration provides only API, only ABI, or both, and `ABIRoleInfo` combines that with a pointer to the counterpart providing the other role (for a declaration that provides both, that’ll just be a pointer to `this`).

Decl checking of behavior specific to @abi will come in a future commit.

Note that this probably doesn’t properly exercise some of the new code (ASTScope::lookupEnclosingABIAttributeScope(), for instance); I expect that to happen only once we can rename types using an @abi attribute, since that will create distinguishable behavior differences when resolving TypeReprs in other @abi attributes.

lib/Serialization/SerializedModuleLoader.cpp

@@ -1851,7 +1851,7 @@ bool SerializedASTFile::isSystemModule() const {
 void SerializedASTFile::lookupValue(DeclName name, NLKind lookupKind,
                                     OptionSet<ModuleLookupFlags> Flags,
                                     SmallVectorImpl<ValueDecl*> &results) const{
-  File.lookupValue(name, results);
+  File.lookupValue(name, Flags, results);
 }
 
 StringRef