[LLVM-C] Use dyn_cast instead of unwrap in LLVMGetDebugLoc functions

Summary:
The `unwrap<Type>` calls can assert with:
```
Assertion failed: (isa<X>(Val) && "cast<Ty>() argument of incompatible type!"), function cast
```
so replace them with `dyn_cast`.

Reviewers: whitequark, abdulras, hiraditya, compnerd

Reviewed By: whitequark

Subscribers: llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D60473

llvm-svn: 359093