Use gimli to get the values of DWARF constants needed by codegen

The `gimli` crate is already a dependency of `thorin-dwp`, which is already a
dependency of `rustc_codegen_ssa`.

compiler/rustc_codegen_llvm/src/debuginfo/mod.rs

@@ -39,6 +39,7 @@ use crate::llvm::debuginfo::{
 use crate::value::Value;
 
 mod create_scope_map;
+mod dwarf_const;
 mod gdb;
 pub(crate) mod metadata;
 mod namespace;