Introduce option to run isl AST generation, but no IR generation.

We replace the options

  -polly-code-generator=none
                       =isl

with the options

  -polly-code-generation=none
                        =ast
                        =full

This allows us to measure the overhead of Polly itself, versus the compile
time increases due to us generating more IR and consequently the LLVM backends
spending more time on this IR.

We also use this opportunity to rename the option. The original name was
introduced at a point where we still had two code generators. CLooG and the
isl AST generator. Since we only have one AST generator left, there is no need
to distinguish between 'isl' and something else. However, being able to disable
code generation all together has been shown useful for debugging. Hence, we
rename and extend this option to make it a good fit for its new use case.

llvm-svn: 280554