Change raw_svector_ostream to reserve the input buffer if necessary, Ted was

Change raw_svector_ostream to reserve the input buffer if necessary, Ted was
right.
 - This class turns out to be much more convenient to use if we do this; clients
   can make sure the buffer is always big enough if they care (since our current
   idiom tends to be to use a SmallString<256> for the input to this we should
   generally be avoiding an unnecessary malloc).

Also, add a convenience raw_svector_ostream::str method which flushes the buffer
and returns a StringRef for the vector contents.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@79446 91177308-0d34-0410-b5e6-96231b3b80d8