bug464969_d_demangle.cpp: Always print "Xa"

bug464969_d_demangle depends on producing an error about an
uninitialized variable. But it prints something different
to stdout based on the value of that variable. Which can
produce failures. Make sure to always print the same thing
to stdout.

memcheck/tests/bug464969_d_demangle.cpp

@@ -4,9 +4,9 @@ extern "C" void _D4test4findFiPxaZPxa()
 {
    int a;
    if (a)
-      std::cout << "a\n";
+      std::cout << "Xa\n";
    else
-      std::cout << "!a\n";
+      std::cout << "X" << "a\n";
 }
 
 int main()

memcheck/tests/bug464969_d_demangle.stdout.exp

@@ -1 +1 @@
-!a
+Xa