Add support for computing SHA1 in LLVM

Provide a class to generate a SHA1 from a sequence of bytes, and
a convenience raw_ostream adaptor.
This will be used to provide a "build-id" by hashing the Module
block when writing bitcode. ThinLTO will use this information for
incremental build.

From: Mehdi Amini <mehdi.amini@apple.com>
llvm-svn: 265094