[llvm-cov] Enforce alignment of function records

Function Records are required to be aligned on 8 bytes. This is enforced for each
records except the first, when one relies on the default alignment within an
std::string. There's no such guarantee, and indeed on 32 bits for some
implementation of std::string this is not enforced.

Provide a portable implementation based on llvm's MemoryBuffer.

Differential Revision: https://reviews.llvm.org/D104745