Set RetainRemappedFileBuffers to true

Reported by David Welch in MaskRay#350.

This fixes double-free of llvm::MemoryBuffer when parsing fails.

src/indexer.cc

@@ -1248,6 +1248,7 @@ Index(SemaManager *manager, WorkingFiles *wfiles, VFS *vfs,
       Clang->getDiagnostics(), Clang->getInvocation().TargetOpts));
   if (!Clang->hasTarget())
     return {};
+  Clang->getPreprocessorOpts().RetainRemappedFileBuffers = true;
 #if LLVM_VERSION_MAJOR >= 9 // rC357037
   Clang->createFileManager(FS);
 #else
@@ -1302,8 +1303,6 @@ Index(SemaManager *manager, WorkingFiles *wfiles, VFS *vfs,
                  << (reason.empty() ? "" : ": " + reason);
     return {};
   }
-  for (auto &Buf : Bufs)
-    Buf.release();
 
   std::vector<std::unique_ptr<IndexFile>> result;
   for (auto &it : param.UID2File) {

src/sema_manager.cc

@@ -302,6 +302,7 @@ std::unique_ptr<CompilerInstance> BuildCompilerInstance(
       Clang->getDiagnostics(), Clang->getInvocation().TargetOpts));
   if (!Clang->hasTarget())
     return nullptr;
+  Clang->getPreprocessorOpts().RetainRemappedFileBuffers = true;
   // Construct SourceManager with UserFilesAreVolatile: true because otherwise
   // RequiresNullTerminator: true may cause out-of-bounds read when a file is
   // mmap'ed but is saved concurrently.
@@ -490,7 +491,6 @@ void *CompletionMain(void *manager_) {
     Clang->setCodeCompletionConsumer(task->Consumer.release());
     if (!Parse(*Clang))
       continue;
-    Buf.release();
 
     task->on_complete(&Clang->getCodeCompletionConsumer());
   }
@@ -580,7 +580,6 @@ void *DiagnosticMain(void *manager_) {
       continue;
     if (!Parse(*Clang))
       continue;
-    Buf.release();
 
     auto Fill = [](const DiagBase &d, Diagnostic &ret) {
       ret.range = lsRange{{d.range.start.line, d.range.start.column},