[llvm-cov gcov] Don't require NUL terminator when reading files

.gcno, .gcda and source files can be modified while we are reading them. If the
concurrent modification of a file being read nullifies the NUL terminator
assumption, llvm-cov can trip over an assertion failure in MemoryBuffer::init.
This is not so rare - the source files can be in an editor and .gcda can be
written by an running process (if the process forks, when .gcda gets written is
probably more unpredictable).

There is no accompanying test because an assertion failure requires data
races with some involved setting.