Assert [[noreturn]] for Log::Fatal to avoid compiler warnings about "sometimes uninitialized"

[MichaelChirico]

clang flags e.g. here under -Wsometimes-uninitialized:

stochtree/src/sampler.cpp

Lines 50 to 56 in 5c09ac7

	StochTree::ModelType model_type;
	if (leaf_model_int == 0) model_type = StochTree::ModelType::kConstantLeafGaussian;
	else if (leaf_model_int == 1) model_type = StochTree::ModelType::kUnivariateRegressionLeafGaussian;
	else if (leaf_model_int == 2) model_type = StochTree::ModelType::kMultivariateRegressionLeafGaussian;
	else if (leaf_model_int == 3) model_type = StochTree::ModelType::kLogLinearVariance;
	else if (leaf_model_int == 4) model_type = StochTree::ModelType::kCloglogOrdinal;
	else StochTree::Log::Fatal("Invalid model type");

That's because the compiler can't tell that Log::Fatal will always throw std::runtime_error(). Using [[noreturn]] is the standard approach here.

https://en.cppreference.com/cpp/language/attributes/noreturn