[llvm] [cmake][llvm] Limit the number of Xcode schemes created by default (PR #101243)

[Jonas Devlieghere via llvm-commits]

JDevlieghere wrote:

+1 on reducing the number of schemes. Thanks for doing this!

I wonder if we could achieve the same result, without iterating over all the targets, by setting `CMAKE_XCODE_GENERATE_SCHEME` to `OFF` and then explicitly turning it on in `llvm_add_tool` and `add_lit_target` (like you did here). 

https://github.com/llvm/llvm-project/pull/101243