Error on insufficient optimization record paths with multi-threaded WMO

In multi-threaded whole module optimization mode, using fewer than N
-save-optimization-record-path arguments for N input files causes
multiple threads to attempt writing to the same file, which is invalid.

This commit adds a diagnostic to catch this error case and provides
guidance to users: they must either specify one path per input file,
use an output file map, or use single-threaded compilation.

Key bug fix: The check now correctly counts actual path arguments
instead of checking for the presence of any optimization record flag.
Previously, the format flag -save-optimization-record=<format> was
incorrectly treated as providing a path, causing false positives.

Author: rjmansfield

Sources/SwiftDriver/Jobs/CompileJob.swift

@@ -271,6 +271,18 @@ extension Driver {
       ? inputs.count
       : primaryInputs.count
 
+    // Check for invalid usage: insufficient optimization record paths with multi-threaded WMO
+    // If explicit paths are provided, need one path per input file
+    // Note: .saveOptimizationRecordEQ is for format (yaml/bitstream), not paths
+    let optRecordPathCount = parsedOptions.arguments(for: .saveOptimizationRecordPath).count
+    let hasOptRecordFileMapEntries = outputFileMap?.hasEntries(for: optimizationRecordFileType ?? .yamlOptimizationRecord) ?? false
+
+    if !compilerMode.usesPrimaryFileInputs && numThreads > 1 && inputs.count > 1 &&
+       optRecordPathCount > 0 && optRecordPathCount < inputs.count && !hasOptRecordFileMapEntries {
+      diagnosticEngine.emit(.error_single_opt_record_path_with_multi_threaded_wmo)
+      throw ErrorDiagnostics.emitted
+    }
+
     outputs += try addFrontendSupplementaryOutputArguments(
       commandLine: &commandLine,
       primaryInputs: primaryInputs,

Sources/SwiftDriver/Utilities/Diagnostics.swift

@@ -182,4 +182,8 @@ extension Diagnostic.Message {
   static var error_no_objc_interop_embedded: Diagnostic.Message {
     .error("Objective-C interop cannot be enabled with embedded Swift.")
   }
+
+  static var error_single_opt_record_path_with_multi_threaded_wmo: Diagnostic.Message {
+    .error("multi-threaded whole-module optimization requires one '-save-optimization-record-path' per source file")
+  }
 }