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/lower/function_context.cpp

@@ -6,6 +6,8 @@
 
 #include "common/vlog.h"
 #include "toolchain/base/kind_switch.h"
+#include "toolchain/base/pretty_stack_trace_function.h"
+#include "toolchain/sem_ir/diagnostic_loc_converter.h"
 #include "toolchain/sem_ir/file.h"
 #include "toolchain/sem_ir/generic.h"
 
@@ -55,7 +57,25 @@ auto FunctionContext::TryToReuseBlock(SemIR::InstBlockId block_id,
 }
 
 auto FunctionContext::LowerBlockContents(SemIR::InstBlockId block_id) -> void {
+  auto inst_id_for_stack_trace = SemIR::InstId::None;
+
+  // On crash, report the instruction we were lowering.
+  PrettyStackTraceFunction stack_trace_entry([&](llvm::raw_ostream& output) {
+    SemIR::DiagnosticLocConverter converter(
+        file_context_->context().tree_and_subtrees_getters(), &sem_ir());
+    auto converted = converter.Convert(SemIR::LocId(inst_id_for_stack_trace),
+                                       /*token_only=*/false);
+    converted.loc.FormatLocation(output);
+    // TODO: Format SemIR for the instruction we were lowering?
+    output << "Lowering "
+           << sem_ir().insts().Get(inst_id_for_stack_trace).kind().ir_name()
+           << "\n";
+    // Crash output has a tab indent; try to indent slightly past that.
+    converted.loc.FormatSnippet(output, /*indent=*/10);
+  });
+
   for (auto inst_id : sem_ir().inst_blocks().Get(block_id)) {
+    inst_id_for_stack_trace = inst_id;
     LowerInst(inst_id);
   }
 }