raw_ostream: If writing a string that is larger than the buffer, write it directly...

raw_ostream: If writing a string that is larger than the buffer, write it directly instead of doing many buffer-sized writes.

This caps the number of write(2) calls per string to a maximum of 2.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@127010 91177308-0d34-0410-b5e6-96231b3b80d8