MSan told me that we actually dump the entire scratch buffer into PCH files, initialize it.

Writing 4k of zeros is preferrable to 4k of random memory. Document that. While
there remove the initialization of the first byte of the buffer and start at
index zero. It was writing a literal '0' instead of a null byte at the
beginning anyways, which didn't matter since we never read it.

llvm-svn: 234202