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.cpp

@@ -16,12 +16,15 @@ namespace Carbon::Lower {
 
 FunctionContext::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)
     : file_context_(&file_context),
       function_(function),
       specific_file_context_(&specific_file_context),
+      specific_sem_ir_function_id_(specific_sem_ir_function_id),
       specific_id_(specific_id),
       builder_(file_context.llvm_context(), llvm::ConstantFolder(),
                Inserter(file_context.inst_namer())),