raw_ostream: while it is generally desirable to do larger writes, it can lead to

raw_ostream: while it is generally desirable to do larger writes, it can lead to
inefficient file system buffering if the writes are not a multiple of the desired
buffer size. Avoid this by limiting the large write to a multiple of the buffer
size and copying the remainder into the buffer.

Thanks to Dan for pointing this out.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@127026 91177308-0d34-0410-b5e6-96231b3b80d8