[lldb][test] TestExprCompletion.py: add tests for completion of reserved identifiers (llvm#84890)

(cherry picked from commit 88bf640)

Author: Michael137

lldb/test/API/commands/expression/completion/Makefile

@@ -1,3 +1,4 @@
 CXX_SOURCES := main.cpp other.cpp
+CXXFLAGS += -isystem $(SRCDIR)/sys
 
 include Makefile.rules

lldb/test/API/commands/expression/completion/TestExprCompletion.py

@@ -246,6 +246,11 @@ def test_expr_completion(self):
             "expr some_expr.Self(). FooNoArgs", "expr some_expr.Self(). FooNoArgsBar()"
         )
 
+        self.complete_from_to("expr myVec.__f", "expr myVec.__func()")
+        self.complete_from_to("expr myVec._F", "expr myVec._Func()")
+        self.complete_from_to("expr myVec.__m", "expr myVec.__mem")
+        self.complete_from_to("expr myVec._M", "expr myVec._Mem")
+
     def test_expr_completion_with_descriptions(self):
         self.build()
         self.main_source = "main.cpp"

lldb/test/API/commands/expression/completion/main.cpp

@@ -1,3 +1,5 @@
+#include <reserved.h>
+
 namespace LongNamespaceName { class NestedClass { long m; }; }
 
 // Defined in other.cpp, we only have a forward declaration here.
@@ -31,5 +33,8 @@ int main()
     some_expr.FooNumbersBar1();
     Expr::StaticMemberMethodBar();
     ForwardDecl *fwd_decl_ptr = &fwd_decl;
+    MyVec myVec;
+    myVec.__func();
+    myVec._Func();
     return 0; // Break here
 }

lldb/test/API/commands/expression/completion/sys/reserved.h

@@ -0,0 +1,8 @@
+class MyVec {
+  int __mem;
+  int _Mem;
+
+public:
+  void __func() {}
+  void _Func() {}
+};