std.builtin: rename Type.Fn's args to params

This was a poor naming choice; these are parameters, not arguments.
Parameters specify what kind of arguments are expected, whereas the arguments are the actual values passed.

test/cases/compile_errors/reify_type.Fn_with_is_generic_true.zig

@@ -5,7 +5,7 @@ const Foo = @Type(.{
         .is_generic = true,
         .is_var_args = false,
         .return_type = u0,
-        .args = &.{},
+        .params = &.{},
     },
 });
 comptime { _ = Foo; }

test/cases/compile_errors/reify_type.Fn_with_is_var_args_true_and_non-C_callconv.zig

@@ -5,7 +5,7 @@ const Foo = @Type(.{
         .is_generic = false,
         .is_var_args = true,
         .return_type = u0,
-        .args = &.{},
+        .params = &.{},
     },
 });
 comptime { _ = Foo; }

test/cases/compile_errors/reify_type.Fn_with_return_type_null.zig

@@ -5,7 +5,7 @@ const Foo = @Type(.{
         .is_generic = false,
         .is_var_args = false,
         .return_type = null,
-        .args = &.{},
+        .params = &.{},
     },
 });
 comptime { _ = Foo; }