raw_svector_ostream: grow and reserve atomically

Including the scratch buffer size in the initial reservation eliminates the
subsequent malloc+move operation and offers a healthier constant growth with
less memory wastage.

When doing this, take care to avoid invalidating the source buffer.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@212816 91177308-0d34-0410-b5e6-96231b3b80d8