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.

Reapply r265094 which was reverted in r265102 because it broke
MSVC bots (constexpr is not supported).

http://reviews.llvm.org/D16325

From: Mehdi Amini <mehdi.amini@apple.com>
llvm-svn: 265107