fmt: keep C. prefix for concrete type used in a generic fn call (#22517)

vlib/v/fmt/fmt.v

@@ -2126,6 +2126,9 @@ fn (mut f Fmt) write_generic_call_if_require(node ast.CallExpr) {
 			} else if tsym.language == .js && !tsym.name.starts_with('JS.') {
 				name = 'JS.' + name
 			}
+			if tsym.language == .c {
+				name = 'C.' + name
+			}
 			f.write(name)
 			f.mark_types_import_as_used(concrete_type)
 			if i != node.concrete_types.len - 1 {

vlib/v/fmt/tests/fn_call_concrete_type_keep.vv

@@ -0,0 +1,16 @@
+fn to_v_array[T](d &T, len u32) []T {
+	mut ret := unsafe { []T{len: int(len)} }
+	for i in 0 .. len {
+		unsafe {
+			ret[i] = d[i]
+		}
+	}
+	unsafe {
+		free(d)
+	}
+	return ret
+}
+
+fn main() {
+	to_v_array[C.Foo](C.Foo{}, 123.2)
+}