[Serialization] Serialize mtime with nanosecond precision

There's no reason to lose accuracy here.

Author: rintaro

lib/Serialization/ModuleFile.cpp

@@ -974,7 +974,7 @@ void ModuleFile::collectBasicSourceFileInfo(
     auto fpStr = StringRef{reinterpret_cast<const char *>(Cursor),
                            Fingerprint::DIGEST_LENGTH};
     Cursor += Fingerprint::DIGEST_LENGTH;
-    // LastModified (seconds since epoch).
+    // LastModified (nanoseconds since epoch).
     auto timestamp = endian::readNext<uint64_t, little, unaligned>(Cursor);
     // FileSize (num of bytes).
     auto fileSize = endian::readNext<uint64_t, little, unaligned>(Cursor);
@@ -987,7 +987,8 @@ void ModuleFile::collectBasicSourceFileInfo(
     BasicSourceFileInfo info;
     info.FilePath = filePath;
     info.InterfaceHash = Fingerprint::fromString(fpStr);
-    info.LastModified = llvm::sys::TimePoint<>(std::chrono::seconds(timestamp));
+    info.LastModified =
+        llvm::sys::TimePoint<>(std::chrono::nanoseconds(timestamp));
     info.FileSize = fileSize;
     callback(info);
   }

lib/Serialization/SerializeDoc.cpp

@@ -804,7 +804,7 @@ static void emitFileListRecord(llvm::BitstreamWriter &Out,
 
       auto fileID = FWriter.getTextOffset(absolutePath);
       auto fingerprintStr = info.InterfaceHash.getRawValue();
-      auto timestamp = std::chrono::duration_cast<std::chrono::seconds>(
+      auto timestamp = std::chrono::duration_cast<std::chrono::nanoseconds>(
                            info.LastModified.time_since_epoch())
                            .count();
 
@@ -815,9 +815,9 @@ static void emitFileListRecord(llvm::BitstreamWriter &Out,
       // InterfaceHash (fixed length string).
       assert(fingerprintStr.size() == Fingerprint::DIGEST_LENGTH);
       out << fingerprintStr;
-      // LastModified.
+      // LastModified (nanoseconds since epoch).
       writer.write<uint64_t>(timestamp);
-      // FileSize.
+      // FileSize (num of bytes).
       writer.write<uint64_t>(info.FileSize);
     }