[NFC] Swap CodeCompletion's lookupQualified for TypeChecker's

This call relied on semantic member synthesis, so now it has to go through TypeChecker::lookupQualified.

include/swift/Sema/IDETypeChecking.h

@@ -35,6 +35,7 @@ namespace swift {
   class Expr;
   class ExtensionDecl;
   class FunctionType;
+  class LookupResult;
   class NominalTypeDecl;
   class PatternBindingDecl;
   class ProtocolDecl;
@@ -137,6 +138,9 @@ namespace swift {
   /// \returns true on success, false on error.
   bool typeCheckTopLevelCodeDecl(TopLevelCodeDecl *TLCD);
 
+  LookupResult
+  lookupSemanticMember(DeclContext *DC, Type ty, DeclName name);
+
   struct ExtensionInfo {
     // The extension with the declarations to apply.
     ExtensionDecl *Ext;