debuginfo: Simplify TypeMap used during LLVM debuginfo generation.

The previous implementation was written before types were properly
normalized for code generation and had to assume a more complicated
relationship between types and their debuginfo -- generating separate
identifiers for debuginfo nodes that were based on normalized types.

Since types are now already normalized, we can use them as identifiers
for debuginfo nodes.

compiler/rustc_codegen_llvm/src/lib.rs

@@ -9,10 +9,14 @@
 #![feature(crate_visibility_modifier)]
 #![feature(let_else)]
 #![feature(extern_types)]
+#![feature(once_cell)]
 #![feature(nll)]
 #![recursion_limit = "256"]
 #![cfg_attr(not(bootstrap), allow(rustc::potential_query_instability))]
 
+#[macro_use]
+extern crate rustc_macros;
+
 use back::write::{create_informational_target_machine, create_target_machine};
 
 pub use llvm_util::target_features;