[llvm] [cmake] Make CMAKE_BUILD_TYPE=Release the default (PR #174520)

[Mehdi Amini via llvm-commits]

joker-eph wrote:

> Going beyond that, our [Getting Started guide](https://llvm.org/docs/GettingStarted.html) is insufficiently opinionated. All the options are documented, but it requires expertise to navigate through the forest to come up with a configuration that makes people productive. There is no perfect build configuration, but I think we should bias the "getting started" docs towards novices, not experts.

I agree, this is how we structured it in MLIR: https://mlir.llvm.org/getting_started/ ; seems like to go in the direction you intend to? 


https://github.com/llvm/llvm-project/pull/174520