[lldb][test] TestCPP20Standard.py: skip on older compilers

Requires C++20 support (at least for the spaceship operator).
Seems to work back to Clang-11

(cherry picked from commit 5379189)

Author: Michael137

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

@@ -4,6 +4,7 @@
 from lldbsuite.test import lldbutil
 
 class TestCPP20Standard(TestBase):
+    @skipIf(compiler="clang", compiler_version=['<', '11.0'])
     def test_cpp20(self):
         """
         Tests that we can evaluate an expression in C++20 mode