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.

Author: michaelwoerister

Diff for: Cargo.lock

@@ -3524,6 +3524,7 @@ dependencies = [
  "rustc_hir",
  "rustc_index",
  "rustc_llvm",
+ "rustc_macros",
  "rustc_metadata",
  "rustc_middle",
  "rustc_query_system",

Diff for: compiler/rustc_codegen_llvm/Cargo.toml

@@ -25,6 +25,7 @@ rustc_fs_util = { path = "../rustc_fs_util" }
 rustc_hir = { path = "../rustc_hir" }
 rustc_index = { path = "../rustc_index" }
 rustc_llvm = { path = "../rustc_llvm" }
+rustc_macros = { path = "../rustc_macros" }
 rustc_metadata = { path = "../rustc_metadata" }
 rustc_query_system = { path = "../rustc_query_system" }
 rustc_session = { path = "../rustc_session" }