Allow overwriting argv with function -a and -V

Previously, if you called a function parameter 'argv', within the body
of the function, argv would be set to *all* the arguments to the
function, and not the one indicated by the parameter name.
The same behaviour happened if you inherited a variable named 'argv'.
Both behaviours were quite surprising, so this commit makes things more
obvious, although they could alternatively simply be made errors.

Part of #11780

tests/checks/function.fish

@@ -192,4 +192,26 @@ end
 # CHECKERR: function ()
 # CHECKERR: ^
 
+# Tests the --argument-names and --inherit-variable can overwrite argv
+function t --argument-names a argv c
+    echo $argv
+end
+t 1 2 3
+#CHECK: 2
+
+function t -a argv
+    echo $argv
+end
+t 1 2 3
+#CHECK: 1
+
+function outer
+    function inner -v argv -V argv
+        echo $argv
+    end
+    set -gx argv 4 5 6
+end
+outer 1 2 3
+#CHECK: 1 2 3
+
 exit 0