[SYCL] Relax device code recursion ban for constexpr calls in constexpr context.

[kbobrovs]

SYCL spec restricts device code not to allow recursion.
In cases when recursion can be evaluated at compile-time, and the restriction can be lifted for programmer convenience. Even though the spec says

Recursion is not allowed in a SYCL kernel or any code called from the kernel

the intent is likely to reliably prevent recursive function code generation for the devices, the spec seems to need clarification.

With the proposed relaxation the following code should compile:

template <typename name, typename Func>
__attribute__((sycl_kernel)) void kernel_single_task(Func kernelFunc) {
  kernelFunc();
}

static constexpr unsigned int getNextPowerOf2(unsigned int n,
                                              unsigned int k = 1) {
  return (k >= n) ? k : getNextPowerOf2(n, k * 2);
}

unsigned test_constexpr_recursion(unsigned int val) {
  unsigned int res = val;
  unsigned int *addr = &res;

  kernel_single_task<class ConstexprRecursionKernel>([=]() {
    // Compiler must evaluate recursion, no errors expected.
    constexpr unsigned int x = getNextPowerOf2(3);
    *addr += x;
  });
  return res;
}

But if getNextPowerOf2 is called from non-constexpr context, it still should not compile:

template <typename name, typename Func>
__attribute__((sycl_kernel)) void kernel_single_task(Func kernelFunc) {
  kernelFunc(); //#call_kernelFunc // expected-note 3{{called by 'kernel_single_task<fake_kernel, (lambda at}}
}

static constexpr unsigned int getNextPowerOf2(unsigned int n,
                                              unsigned int k = 1) {
  return (k >= n) ? k : getNextPowerOf2(n, k * 2);
}

unsigned test_constexpr_recursion(unsigned int val) {
  unsigned int res = val;
  unsigned int *addr = &res;

  kernel_single_task<class ConstexprRecursionKernel>([=]() {
    // This call should still generate an error, as getNextPowerOf2
    //  will not be compile-time evaluated in this case and will be code-generated:
    unsigned int x = getNextPowerOf2(3);
    *addr += x;
  });
  return res;
}

[erichkeane]

Note this is actually more complicated than just when assigning to a constexpr variable. Things like, the condition of a static assert, "constexpr if", array bounds, etc (basically, anywhere a constant expression is required!) should not diagnose if we are to allow this.

[KornevNikita]

Done by #2105.