[libc] Added checks to src and dest types in bit_cast

This patch adds assertions to prevent the compilation when we try to
bit cast a type that is not trivially copyable when using
__builtin_bit_cast, or when we try to bit cast a type that is not
trivially copyable and trivially constructable when using memcpy.

Reviewed By: sivachandra

Differential Revision: https://reviews.llvm.org/D148739