Convert LLVMBool parameteter/result to Ruby boolean

Convert LLVMBool parameteter/result to Ruby boolean in these functions:

* `LLVM::GlobalValue#global_constant?`
* `LLVM::GlobalValue#global_constant=`
* `LLVM::PassManagerBuilder#build_with_lto`

Note: This breaks backward compatibility. The code `value.global_constant = 0`, for example, will **turn on** global_constant.