[CMake] Enable '-index-store-path' compiler option on a Debug build, if the compiler supports it and for non-IDE generators
[CMake] Enable '-index-store-path' compiler option on a Debug build, if the compiler supports it and for non-IDE generators

It can also be explicitely disabled via `LLVM_DISABLE_INDEX_STORE`.

apple/swift-llvm,apple/swift-llvm,apple/swift-llvm,apple/swift-llvm,apple/swift-llvm,apple/swift-llvm,apple/swift-llvm,apple/swift-llvm