[lldb][test] TestCPP20Standard.py: make it a libc++ test

We just want to test whether the language switch works.
This is easier to control for libc++, since for bots building
the tests against libstdc++ we might not have the necessary
`<compare>` header available currently.

Author: Michael137

lldb/test/API/lang/cpp/standards/cpp20/Makefile

@@ -1,3 +1,4 @@
+USE_LIBCPP := 1
 CXX_SOURCES := main.cpp
 CXXFLAGS_EXTRAS := -std=c++20
 

lldb/test/API/lang/cpp/standards/cpp20/TestCPP20Standard.py

@@ -4,6 +4,7 @@
 from lldbsuite.test import lldbutil
 
 class TestCPP20Standard(TestBase):
+    @add_test_categories(["libc++"])
     @skipIf(compiler="clang", compiler_version=['<', '11.0'])
     def test_cpp20(self):
         """