Update to C++20

It's time to update to the new standard. It has nice features we can
make use of.

Signed-off-by: Philipp Schrader <philipp.schrader@gmail.com>
Change-Id: Idb125d2239771972f0c3bfde221f24b18ddfb41f
diff --git a/WORKSPACE b/WORKSPACE
index f98a2da..8db3b72 100644
--- a/WORKSPACE
+++ b/WORKSPACE
@@ -290,7 +290,7 @@
 ]
 
 llvm_cxxopts = [
-    "-std=gnu++17",
+    "-std=gnu++20",
 ]
 
 llvm_opt_copts = [