Improve backtrace for lowering crashes. (#5651)

Factor out the logic for mapping from a `LocId` into a diagnostic
location from check into sem_ir so it can be reused by lowering. Include
the function and instruction being lowered in the pretty stack trace.
Example stack trace:

```carbon
2.      filename: examples/sieve.carbon
3.      core/prelude/types/int.carbon:213:3: lowering function Core.Op(Core.IntLiteral as Core.ImplicitAs(i32))
            fn Op[addr self: Self*](other: Self) = "int.sadd_assign";
            ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
4.      core/prelude/operators/arithmetic.carbon:22:27: lowering call
            fn Op[addr self: Self*](other: Other);
                                    ^~~~~~~~~~~~
```

toolchain/driver/compile_subcommand.cpp

@@ -708,13 +708,11 @@ auto CompilationUnit::RunLower() -> void {
     // TODO: Consider disabling instruction naming by default if we're not
     // producing textual LLVM IR.
     SemIR::InstNamer inst_namer(&*sem_ir_);
-    std::optional<llvm::ArrayRef<Parse::GetTreeAndSubtreesFn>> subtrees;
-    if (options_->include_debug_info) {
-      subtrees = cache_->tree_and_subtrees_getters();
-    }
-    module_ = Lower::LowerToLLVM(*llvm_context_, driver_env_->fs, subtrees,
-                                 input_filename_, *sem_ir_, &inst_namer,
-                                 vlog_stream_);
+    llvm::ArrayRef<Parse::GetTreeAndSubtreesFn> subtrees =
+        cache_->tree_and_subtrees_getters();
+    module_ = Lower::LowerToLLVM(
+        *llvm_context_, driver_env_->fs, options_->include_debug_info, subtrees,
+        input_filename_, *sem_ir_, &inst_namer, vlog_stream_);
   });
   if (vlog_stream_) {
     CARBON_VLOG("*** llvm::Module ***\n");
@@ -846,7 +844,7 @@ auto CompilationUnit::LogCall(llvm::StringLiteral logging_label,
                               llvm::StringLiteral timing_label,
                               llvm::function_ref<auto()->void> fn) -> void {
   PrettyStackTraceFunction trace_file([&](llvm::raw_ostream& out) {
-    out << "filename: " << input_filename_ << "\n";
+    out << "Filename: " << input_filename_ << "\n";
   });
   CARBON_VLOG("*** {0}: {1} ***\n", logging_label, input_filename_);
   Timings::ScopedTiming timing(timings_ ? &*timings_ : nullptr, timing_label);