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

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.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@107758 91177308-0d34-0410-b5e6-96231b3b80d8