add a new raw_ostream class which is an extremely high performance ostream that

can *only* output data (no seeking, reading, etc).  This is adapted from the
clang "-E outputter", and is roughly 10% faster than stdio on darwin and 30%
(or more) faster than std::ostream.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@54855 91177308-0d34-0410-b5e6-96231b3b80d8