1. Don't force debug builds to have assertion checking turned on always.
   Let the default (on) be taken, or overridden by the command line
2. Create two new BuildModes based on assertion checking: Release+Assert
   and Debug-Assert.
3. Ensure that when building a distribution we get a release build with
   assertions enabled, regardless of the tree's configuration.
4. (unrelated) Fix library name generation for llvm-config usage.

llvm-svn: 27488