Add direct operator lookup requests

Introduce DirectOperatorLookupRequest &
DirectPrecedenceGroupLookupRequest that lookup
operator and precedence groups within a given
file or module without looking through imports.

These will eventually be used as the basis for the
new operator lookup implementation, but for now
just use them when querying lookup results from
serialized module files.

include/swift/Serialization/SerializedModuleLoader.h

@@ -328,12 +328,16 @@ public:
   lookupNestedType(Identifier name,
                    const NominalTypeDecl *parent) const override;
 
-  virtual OperatorDecl *lookupOperator(Identifier name,
-                                       OperatorFixity fixity) const override;
+protected:
+  virtual void
+  lookupOperatorDirect(Identifier name, OperatorFixity fixity,
+                       TinyPtrVector<OperatorDecl *> &results) const override;
 
-  virtual PrecedenceGroupDecl *
-  lookupPrecedenceGroup(Identifier name) const override;
+  virtual void lookupPrecedenceGroupDirect(
+      Identifier name,
+      TinyPtrVector<PrecedenceGroupDecl *> &results) const override;
 
+public:
   virtual void lookupVisibleDecls(ModuleDecl::AccessPathTy accessPath,
                                   VisibleDeclConsumer &consumer,
                                   NLKind lookupKind) const override;