[CMake] Name the bootstrap stages stage[0-9]

When you start chaining bootstrap stages the CMake-generated targets get unwieldy. This change supports naming the bootstrap targets and creating wrapper targets in the top-level build file.

Without this patch the default target generated for a second stage build was "bootstrap" with it the target is "stage2".

Also with some CMake goop setting CLANG_BOOTSTRAP_TARGETS, you can expose third stage targets as "stage3" instead of "bootstrap-bootstrap"

llvm-svn: 255813