oneAPI supports cxx17_flag and cxx20_flag (#31274)

lib/spack/spack/compilers/oneapi.py

@@ -81,6 +81,14 @@ def cxx11_flag(self):
     def cxx14_flag(self):
         return "-std=c++14"
 
+    @property
+    def cxx17_flag(self):
+        return "-std=c++17"
+
+    @property
+    def cxx20_flag(self):
+        return "-std=c++20"
+
     @property
     def c99_flag(self):
         return "-std=c99"