Excise the global TranslationUnit from TypeChecker.

Now that TypeChecker is being used to check all sorts of things (not all
from a single TU), it's not really correct to have a single top-level TU
that gets used everywhere. Instead, we should be using the TU that's
appropriate for whatever's being checked. This is a small correctness win
for order-independent type-checking, but is critical for multi-file
translation units, which is needed for implicit visibility.

This basically involves passing around DeclContexts much more.

Caveat: we aren't smart about, say, filtering extensions based on the
current context, so we're still not 100% correct here.

Swift SVN r9006

lib/AST/Module.cpp

@@ -560,8 +560,8 @@ specializeTypeWitnesses(ASTContext &ctx,
 /// Retrieve the explicit conformance of the given nominal type declaration
 /// to the given protocol.
 static std::tuple<NominalTypeDecl *, Decl *, ProtocolConformance *>
-findExplicitConformance(Module *module, NominalTypeDecl *nominal,
-                        ProtocolDecl *protocol, LazyResolver *resolver) {
+findExplicitConformance(NominalTypeDecl *nominal, ProtocolDecl *protocol,
+                        LazyResolver *resolver) {
   // FIXME: Introduce a cache/lazy lookup structure to make this more efficient?
 
   // Walk the nominal type, its extensions, superclasses, and so on.
@@ -726,7 +726,7 @@ LookupConformanceResult Module::lookupConformance(Type type,
   Decl *declaresConformance = nullptr;
   ProtocolConformance *nominalConformance = nullptr;
   std::tie(owningNominal, declaresConformance, nominalConformance)
-    = findExplicitConformance(this, nominal, protocol, resolver);
+    = findExplicitConformance(nominal, protocol, resolver);
 
   // If we didn't find an owning nominal, we don't conform. Cache the negative
   // result and return.