Add way to get a `GlobalValue` from a `PointerValue`

[programmerjake]

Is your feature request related to a problem? Please describe.

I have not found a way to get a GlobalValue from a PointerValue without using unsafe { GlobalValue::new(...) }

Describe the solution you'd like
Add some nice conversion methods, maybe also an enum that covers all LLVM types that can be a LLVMValueRef.

Describe possible drawbacks to your solution
None

Describe alternatives you've considered
Just use unsafe, or have a HashMap from all GlobalValues to the corresponding PointerValues.

[TheDan64]

I think part of the issue is that All GlobalValues are PointerValues but not all PointerValues are GlobalValues. IIRC llvm, at least the C api, doesn't give us a way to distinguish the two types for a fallible conversion

[programmerjake]

Just use LLVMIsAGlobalVariable, since it seems like inkwell's GlobalValue really means LLVM's GlobalVariable.
For the whole list of available checks, see https://docs.rs/llvm-sys/201.0.1/llvm_sys/core/index.html?search=LLVMIsA

[TheDan64]

ahhhh good catch. I would support that fallible conversion with a TryFrom impl then