[Frontend] Verify that the bitstream is not empty before reading the serialised diagnostics

[arphaman]

Summary

Clang should avoid calling report_fatal_error when the file with the serialised diagnostics is empty. This patch changes Clang's serialised diagnostic reader, now it reports an appropriate error instead of crashing.

[dexonsmith]

LGTM.

cfe/trunk/lib/Frontend/SerializedDiagnosticReader.cpp

std::error_code SerializedDiagnosticReader::readDiagnostics(StringRef File) {
 
   auto Buffer = FileMgr.getBufferForFile(File);
   if (!Buffer)
     return SDError::CouldNotLoad;
 
   llvm::BitstreamCursor Stream(**Buffer);
   Optional<llvm::BitstreamBlockInfo> BlockInfo;
 
+  if (Stream.AtEndOfStream())
+    return SDError::InvalidSignature;
+
   // Sniff for the signature.
   if (Stream.Read(8) != 'D' ||
       Stream.Read(8) != 'I' ||
       Stream.Read(8) != 'A' ||
       Stream.Read(8) != 'G')
     return SDError::InvalidSignature;
 
   // Read the top level blocks.

cfe/trunk/test/Index/read-empty-diags.test

+// RUN: not c-index-test -read-diagnostics %S/Inputs/empty.dia 2>&1 | FileCheck %s
+// CHECK: Trouble deserializing file (Invalid File): Invalid diagnostics signature