[clang][index] Index unresolved member expression as reference

Unresolved member expressions aren't indexed as references.

Example code:

```
struct Foo {
  template <typename T> void bar(T t);
};
template <typename T> void test(Foo F, T t) {
  F.bar(t); // Not indexed
}
```

Reviewed By: hokein

Differential Revision: https://reviews.llvm.org/D131091

clang-tools-extra/clangd/unittests/XRefsTests.cpp

@@ -2090,6 +2090,14 @@ TEST(FindReferences, WithinAST) {
           [[f^oo]](s);
         }
       )cpp",
+      R"cpp(// unresolved member expression
+        struct Foo {
+          template <typename T> void $decl[[b^ar]](T t); 
+        };
+        template <typename T> void test(Foo F, T t) {
+          F.[[bar]](t);
+        }
+      )cpp",
 
       // Enum base
       R"cpp(

clang/lib/Index/IndexBody.cpp

@@ -468,7 +468,7 @@ public:
     return true;
   }
 
-  bool VisitUnresolvedLookupExpr(UnresolvedLookupExpr *E) {
+  bool VisitOverloadExpr(OverloadExpr *E) {
     SmallVector<SymbolRelation, 4> Relations;
     SymbolRoleSet Roles = getRolesForRef(E, Relations);
     for (auto *D : E->decls())

clang/test/Index/Core/index-dependent-source.cpp

@@ -231,3 +231,12 @@ template <typename T> void bar() {
   foo<T>();
 // CHECK: [[@LINE-1]]:3 | function/C | foo | c:@FT@>1#Tfoo#v# | <no-cgname> | Ref,Call,RelCall,RelCont | rel: 1
 }
+
+struct Foo {
+  template <typename T> void bar();
+  // CHECK: [[@LINE-1]]:30 | instance-method/C++ | bar | c:@S@Foo@FT@>1#Tbar#v# | <no-cgname> | Decl,RelChild | rel: 1
+};
+template <typename T> void baz(Foo f) {
+  f.bar<T>();
+  // CHECK: [[@LINE-1]]:5 | instance-method/C++ | bar | c:@S@Foo@FT@>1#Tbar#v# | <no-cgname> | Ref,Call,RelCall,RelCont | rel: 1
+}