c++: Add -std=gnu++20 option [PR93958]

One missing bit from r10-6656.  The docs and target-supports.exp
already handle -std=gnu++20.

2020-03-02  Marek Polacek  <polacek@redhat.com>

	PR c++/93958 - add missing -std=gnu++20.
	* c.opt: Add -std=gnu++20.

gcc/c-family/ChangeLog

+2020-03-02  Marek Polacek  <polacek@redhat.com>
+
+	PR c++/93958 - add missing -std=gnu++20.
+	* c.opt: Add -std=gnu++20.
+
 2020-03-01  Martin Sebor  <msebor@redhat.com>
 
 	PR c++/92721

gcc/c-family/c.opt

@@ -2149,7 +2149,11 @@ Conform to the ISO 2017 C++ standard with GNU extensions.
 
 std=gnu++2a
 C++ ObjC++
-Conform to the ISO 2020(?) C++ draft standard with GNU extensions (experimental and incomplete support).
+Conform to the ISO 2020 C++ draft standard with GNU extensions (experimental and incomplete support).
+
+std=gnu++20
+C++ ObjC++ Alias(std=gnu++2a)
+Conform to the ISO 2020 C++ draft standard with GNU extensions (experimental and incomplete support).
 
 std=gnu11
 C ObjC