Refactor resolving a location into a SemIR library (#4876)

At present, lower depends on `Check::SemIRDiagnosticConverter` for debug
info. That was to support a quick implementation of debug info, but
isn't great because it's both an unusual dependency on check's
implementation, and relying on diagnostic structures for debug info.

This cleans that up by splitting relevant logic out to a library in
sem_ir, and having lowering use sem_ir's library instead of check's.
Additionally, a small refactoring of `Parse::TreeAndSubtrees` to allow
getting locations in lowering without going through a `DiagnosticLoc`.
I'm adding `Parse::GetTreeAndSubtreesFn` in because it's a complex
signature to have in so many spots.

I chose to have `ResolveNodeId` return a `SmallVector` because it seemed
likely to be fairly compact, but that could also be using an optional
callback to handle resolved node IDs, possibly just returning the last
entry. This could be switched if preferred.

Note this change shouldn't affect behavior, it's just moving code
around.

---------

Co-authored-by: Chandler Carruth <chandlerc@gmail.com>

toolchain/driver/compile_subcommand.cpp

@@ -335,19 +335,20 @@ class CompilationUnit {
 
   // Prepares per-IR lazy fetch functions which may come up in cross-IR
   // diagnostics.
-  auto PreCheck() -> llvm::function_ref<const Parse::TreeAndSubtrees&()>;
+  auto PreCheck() -> Parse::GetTreeAndSubtreesFn;
 
   // Returns information needed to check this unit.
   auto GetCheckUnit(
       SemIR::CheckIRId check_ir_id,
-      llvm::ArrayRef<llvm::function_ref<const Parse::TreeAndSubtrees&()>>
-          all_trees_and_subtrees) -> Check::Unit;
+      llvm::ArrayRef<Parse::GetTreeAndSubtreesFn> all_trees_and_subtrees)
+      -> Check::Unit;
 
   // Runs post-check logic. Returns true if checking succeeded for the IR.
   auto PostCheck() -> void;
 
   // Lower SemIR to LLVM IR.
-  auto RunLower() -> void;
+  auto RunLower(std::optional<llvm::ArrayRef<Parse::GetTreeAndSubtreesFn>>
+                    all_trees_and_subtrees_for_debug_info) -> void;
 
   auto RunCodeGen() -> void;
 
@@ -499,8 +500,7 @@ auto CompilationUnit::RunParse() -> void {
   }
 }
 
-auto CompilationUnit::PreCheck()
-    -> llvm::function_ref<const Parse::TreeAndSubtrees&()> {
+auto CompilationUnit::PreCheck() -> Parse::GetTreeAndSubtreesFn {
   CARBON_CHECK(parse_tree_, "Must call RunParse first");
   CARBON_CHECK(!get_parse_tree_and_subtrees_, "Called PreCheck twice");
 
@@ -512,8 +512,8 @@ auto CompilationUnit::PreCheck()
 
 auto CompilationUnit::GetCheckUnit(
     SemIR::CheckIRId check_ir_id,
-    llvm::ArrayRef<llvm::function_ref<const Parse::TreeAndSubtrees&()>>
-        all_trees_and_subtrees) -> Check::Unit {
+    llvm::ArrayRef<Parse::GetTreeAndSubtreesFn> all_trees_and_subtrees)
+    -> Check::Unit {
   CARBON_CHECK(get_parse_tree_and_subtrees_, "Must call PreCheck first");
   CARBON_CHECK(!sem_ir_converter_, "Called GetCheckUnit twice");
 
@@ -587,15 +587,17 @@ auto CompilationUnit::PostCheck() -> void {
   }
 }
 
-auto CompilationUnit::RunLower() -> void {
+auto CompilationUnit::RunLower(
+    std::optional<llvm::ArrayRef<Parse::GetTreeAndSubtreesFn>>
+        all_trees_and_subtrees_for_debug_info) -> void {
   LogCall("Lower::LowerToLLVM", "lower", [&] {
     llvm_context_ = std::make_unique<llvm::LLVMContext>();
     // TODO: Consider disabling instruction naming by default if we're not
     // producing textual LLVM IR.
     SemIR::InstNamer inst_namer(&*sem_ir_);
-    module_ = Lower::LowerToLLVM(*llvm_context_, options_.include_debug_info,
-                                 *sem_ir_converter_, input_filename_, *sem_ir_,
-                                 &inst_namer, vlog_stream_);
+    module_ = Lower::LowerToLLVM(
+        *llvm_context_, all_trees_and_subtrees_for_debug_info, input_filename_,
+        *sem_ir_, &inst_namer, vlog_stream_);
   });
   if (vlog_stream_) {
     CARBON_VLOG("*** llvm::Module ***\n");
@@ -842,8 +844,7 @@ auto CompileSubcommand::Run(DriverEnv& driver_env) -> DriverResult {
   }
 
   // Pre-check assigns IR IDs and constructs node converters.
-  llvm::SmallVector<llvm::function_ref<const Parse::TreeAndSubtrees&()>>
-      all_trees_and_subtrees;
+  llvm::SmallVector<Parse::GetTreeAndSubtreesFn> all_trees_and_subtrees;
   // This size may not match due to units that are missing source, but that's an
   // error case and not worth extra work.
   all_trees_and_subtrees.reserve(units.size());
@@ -887,8 +888,13 @@ auto CompileSubcommand::Run(DriverEnv& driver_env) -> DriverResult {
   }
 
   // Lower.
+  std::optional<llvm::ArrayRef<Parse::GetTreeAndSubtreesFn>>
+      all_trees_and_subtrees_for_debug_info;
+  if (options_.include_debug_info) {
+    all_trees_and_subtrees_for_debug_info = all_trees_and_subtrees;
+  }
   for (const auto& unit : units) {
-    unit->RunLower();
+    unit->RunLower(all_trees_and_subtrees_for_debug_info);
   }
   if (options_.phase == CompileOptions::Phase::Lower) {
     return make_result();