[lldb][Modules] Make decls from submodules visible for name lookup

lldb/source/Plugins/ExpressionParser/Clang/ClangModulesDeclVendor.cpp

@@ -379,6 +379,13 @@ bool ClangModulesDeclVendorImpl::AddModule(const SourceModule &module,
     }
   }
 
+  // If we didn't make the submodule visible here, Clang wouldn't allow LLDB to
+  // pick any of the decls in the submodules during C++ name lookup.
+  if (submodule)
+    m_compiler_instance->makeModuleVisible(
+        submodule, clang::Module::NameVisibilityKind::AllVisible,
+        /*ImportLoc=*/{});
+
   clang::Module *requested_module = DoGetModule(clang_path, true);
 
   if (requested_module != nullptr) {

lldb/test/API/lang/cpp/decl-from-submodule/TestDeclFromSubmodule.py

@@ -11,17 +11,11 @@
 class DeclFromSubmoduleTestCase(TestBase):
     # Requires DWARF debug info which is not retained when linking with link.exe.
     @skipIfWindows
+    # Lookup for decls in submodules fails in Linux
+    @expectedFailureAll(oslist=["linux"])
     def test_expr(self):
         self.build()
         lldbutil.run_to_source_breakpoint(self, "return 0", lldb.SBFileSpec("main.cpp"))
 
-        # FIXME: LLDB finds the decl for 'func' in the submodules correctly and hands it to Clang
-        # but Sema rejects using the decl during name lookup because it is not marked "Visible".
-        # However, this assertions still ensures that we at least don't fail to compile the
-        # submodule (which would cause other errors to appear before the expression error, hence
-        # we use "startstr").
-        self.expect(
-            "expr func(1, 2)",
-            error=True,
-            startstr="error: <user expression 0>:1:1: 'func' has unknown return type",
-        )
+        self.expect_expr("func(1, 2)", result_type="int", result_value="3")
+        self.expect_expr("func(1)", result_type="int", result_value="1")