[C++] Enhance functions in util/ubsan.h to support types that do not have a default constructor

[andishgar]

Describe the enhancement requested

Currently, the functions in util/ubsan.h require a type that is trivially copyable. However, before performing its intended operation, it calls the default constructor of the type, as shown in the code below:

arrow/cpp/src/arrow/util/ubsan.h

Lines 56 to 61 in b2e8f25

	inline std::enable_if_t<std::is_trivially_copyable_v<T>, T> SafeLoadAs(
	const uint8_t* unaligned) {
	std::remove_const_t<T> ret;
	std::memcpy(&ret, unaligned, sizeof(T));
	return ret;
	}

It is possible, however, for a type to be trivially copyable without having a default constructor, for example:

struct A {
  A(int a) : a(a) {}
  int a;
};

Component(s)

C++

[pitrou]

Issue resolved by pull request 48429
#48429