Fix for OpenMP offloading compilation error with GNU++20 option when using complex header (llvm#115306)

The change done is to fix this issue.
[[Issue](llvm#113207)] discussion
available in link provided.

When using the -std=c++20 flag with -fopenmp for OpenMP offloading
results in the following compilation error.

Reduced test Case :
```
> cat foo.cpp
#include <complex>

void foo(){
}
> CC -fopenmp -std=gnu++20 -fopenmp-targets=amdgcn-amd-amdhsa -Xopenmp-target=amdgcn-amd-amdhsa -march=gfx90a foo.cpp -c
In file included from foo.cpp:1:
In file included from /opt/cray/pe/cce/18.0.1/cce-clang/x86_64/lib/clang/18/include/openmp_wrappers/complex:51:
/opt/cray/pe/cce/18.0.1/cce-clang/x86_64/lib/clang/18/include/openmp_wrappers/complex_cmath.h:68:40: error: non-constexpr declaration of 'conj' follows constexpr declaration
   68 | template <class _Tp> std::complex<_Tp> conj(const std::complex<_Tp> &__c) {
      |                                        ^
/usr/lib64/gcc/x86_64-suse-linux/13/../../../../include/c++/13/complex:970:5: note: previous declaration is here
  970 |     conj(const complex<_Tp>& __z)
      |     ^
1 error generated.
```

Co-authored-by: Chandra Ghale <[email protected]>

Author: 2 people

clang/lib/Headers/openmp_wrappers/complex_cmath.h

@@ -64,8 +64,13 @@ template <class _Tp> __DEVICE__ _Tp norm(const std::complex<_Tp> &__c) {
 }
 
 // conj
-
-template <class _Tp> std::complex<_Tp> conj(const std::complex<_Tp> &__c) {
+#ifdef _GLIBCXX20_CONSTEXPR
+#define CXX20_CONSTEXPR_DEVICE __DEVICE__
+#else
+#define CXX20_CONSTEXPR_DEVICE
+#endif
+template <class _Tp>
+CXX20_CONSTEXPR_DEVICE std::complex<_Tp> conj(const std::complex<_Tp> &__c) {
   return std::complex<_Tp>(__c.real(), -__c.imag());
 }