return 0 from Run when it doesn't specify a return value (#7180)

https://carbon.compiler-explorer.com/z/88K9Kh5Wo shows the program
exiting with a garbage value copied from uninitialized memory.

This PR modifies `lower` to detect if the function lowered is the
entry point and doesn't specify a return type. If so, it emits
different LLVM IR to return int32 0, and modifies the lowered
function signature to match the int32 return type.

toolchain/lower/function_context.h

@@ -27,7 +27,9 @@ class FunctionContext {
   // be null (see members).
   explicit FunctionContext(
       FileContext& file_context, llvm::Function* function,
-      FileContext& specific_file_context, SemIR::SpecificId specific_id,
+      FileContext& specific_file_context,
+      SemIR::FunctionId specific_sem_ir_function_id,
+      SemIR::SpecificId specific_id,
       SpecificCoalescer::SpecificFunctionFingerprint* function_fingerprint,
       llvm::DISubprogram* di_subprogram, llvm::raw_ostream* vlog_stream);
 
@@ -242,6 +244,11 @@ class FunctionContext {
   auto specific_sem_ir() -> const SemIR::File& {
     return specific_file_context_->sem_ir();
   }
+
+  auto specific_sem_ir_function_id() -> SemIR::FunctionId {
+    return specific_sem_ir_function_id_;
+  }
+
   // The specific ID for the function that is being lowered. Note that this is
   // an ID from `specific_sem_ir()`, not from `sem_ir()`.
   auto specific_id() -> SemIR::SpecificId { return specific_id_; }
@@ -313,6 +320,9 @@ class FunctionContext {
   // defined in a different file.
   FileContext* specific_file_context_;
 
+  // The function id of the function we're lowering.
+  SemIR::FunctionId specific_sem_ir_function_id_;
+
   // The specific id, if the function is a specific.
   SemIR::SpecificId specific_id_;