[OpenMP] Allow exceptions in target regions when offloading to GPUs

The motivation for this patch is that many code bases use exception handling. As GPUs are not expected to support exception handling in the near future, we can experiment with compiling the code for GPU targets anyway. This will
allow us to run the code, as long as no exception is thrown.

The overall idea is very simple:
- If a throw expression is compiled to AMDGCN or NVPTX, it is replaced with a trap during code generation.
- If a try/catch statement is compiled to AMDGCN or AMDHSA, we ganerate code for the try statement as if it were a basic block.

With this patch, the compilation of the following example
```{C++}
int gaussian_sum(int a,int b){
	if ((a + b) % 2 == 0) {throw -1;};
	return (a+b) * ((a+b)/2);
}

int main(void) {
	int gauss = 0;
	#pragma omp target map(from:gauss)
	{
		try {
			gauss = gaussian_sum(1,100);
		}
		catch (int e){
			gauss = e;
		}
	}
	std::cout << "GaussianSum(1,100)="<<gauss<<std::endl;
        #pragma omp target map(from:gauss)
        {
                try {
                     	gauss = gaussian_sum(1,101);
                }
                catch (int e){
                        gauss = e;
                }
        }
	std::cout << "GaussianSum(1,101)="<<gauss<<std::endl;
	return (gauss > 1) ? 0 : 1;
}
```
with offloading to `gfx906` results in
```{bash}
./bin/target_try_minimal_fail
GaussianSum(1,100)=5050
AMDGPU fatal error 1: Received error in queue 0x155555506000: HSA_STATUS_ERROR_EXCEPTION: An HSAIL operation resulted in a hardware exception.
zsh: abort (core dumped)
```

Reviewed By: jdoerfert

Differential Revision: https://reviews.llvm.org/D153924