[Serialization] Add warning when .swiftsourceinfo is malformed

Output a warning rather than ignoring a malformed .swiftsourceinfo
completely.

Resolves rdar://77350048

lib/Serialization/ModuleFileSharedCore.h

@@ -518,6 +518,13 @@ public:
   /// Returns \c true if this module file contains a section with incremental
   /// information.
   bool hasIncrementalInfo() const { return HasIncrementalInfo; }
+
+  /// Returns \c true if a corresponding .swiftsourceinfo has been found.
+  bool hasSourceInfoFile() const { return !!ModuleSourceInfoInputBuffer; }
+
+  /// Returns \c true if a corresponding .swiftsourceinfo has been found *and
+  /// read*.
+  bool hasSourceInfo() const;
 };
 
 template <typename T, typename RawData>