cgen: add requires to c_reserved, so that V generated code, can be compiled with -cc g++-11 -no-std -cflags -std=c++20 as well (#17404)

.github/workflows/other_ci.yml

@@ -81,17 +81,20 @@ jobs:
           sudo apt-get update
           sudo apt-get install --quiet -y libsodium-dev libssl-dev sqlite3 libsqlite3-dev postgresql libpq-dev valgrind
           sudo apt-get install --quiet -y libfreetype6-dev libxi-dev libxcursor-dev libgl-dev xfonts-75dpi xfonts-base
-          sudo apt-get install --quiet -y g++-9
+          sudo apt-get install --quiet -y g++-9 g++-11
 
       - name: Build v
         run: make
 
-      - name: g++ version
+      - name: g++-9 version
         run: g++-9 --version
-      - name: V self compilation with g++
+      - name: V self compilation with g++ and -std=c++11
         run: ./v -cc g++-9 -no-std -cflags -std=c++11 -o v2 cmd/v && ./v2 -cc g++-9 -no-std -cflags -std=c++11 -o v3 cmd/v
-        ##    - name: Running tests with g++
-        ##      run: ./v -cc g++-9 test-self
+
+      - name: g++-11 version
+        run: g++-11 --version
+      - name: V self compilation with g++ and -std=c++20
+        run: ./v -cc g++-11 -no-std -cflags -std=c++20 -o v2 cmd/v && ./v2 -cc g++-11 -no-std -cflags -std=c++20 -o v3 cmd/v
 
       - name: Ensure V can be compiled with -autofree
         run: ./v -autofree -o v2 cmd/v  ## NB: this does not mean it runs, but at least keeps it from regressing
@@ -150,4 +153,3 @@ jobs:
           ./v test-parser -S examples/cli_fuzz.v
           ./v test-parser -S examples/regex_example_fuzz.v
           ./v test-parser -S examples/2048/2048_fuzz.v
-