Rename "Release" builds as "Release+Asserts"; rename "Release-Asserts"

builds to "Release".  The default build is unchanged (optimization on,
assertions on), however it is now called Release+Asserts.  The intent
is that future LLVM releases released via llvm.org will be Release builds
in the new sense, i.e. will have assertions disabled (currently they have
assertions enabled, for a more than 20% slowdown).  This will bring them
in line with MacOS releases, which ship with assertions disabled.  It also
means that "Release" now means the same things in make and cmake builds:
cmake already disables assertions for "Release" builds AFAICS.

llvm-svn: 107758