WIP: Speculative fix for ASTImporter crash with SLocEntry changes

[dexonsmith]

Summary

This is a speculative fix for the ASTImporter crash in https://reviews.llvm.org/D89749; I will clean this up and commit if it's the right direction.

clang/lib/AST/ASTImporter.cpp

Expected<FileID> ASTImporter::Import(FileID FromID, bool IsBuiltin) {
 
   SourceManager &FromSM = FromContext.getSourceManager();
   SourceManager &ToSM = ToContext.getSourceManager();
   const SrcMgr::SLocEntry &FromSLoc = FromSM.getSLocEntry(FromID);
 
   // Map the FromID to the "to" source manager.
   FileID ToID;
   if (FromSLoc.isExpansion()) {
-    const SrcMgr::ExpansionInfo &FromEx = FromSLoc.getExpansion();
+    SrcMgr::ExpansionInfo FromEx = FromSLoc.getExpansion();
     ExpectedSLoc ToSpLoc = Import(FromEx.getSpellingLoc());
     if (!ToSpLoc)
       return ToSpLoc.takeError();
     ExpectedSLoc ToExLocS = Import(FromEx.getExpansionLocStart());
     if (!ToExLocS)
       return ToExLocS.takeError();
     unsigned TokenLen = FromSM.getFileIDSize(FromID);
     SourceLocation MLoc;
     if (FromEx.isMacroArgExpansion()) {
       MLoc = ToSM.createMacroArgExpansionLoc(*ToSpLoc, *ToExLocS, TokenLen);
     } else {
       if (ExpectedSLoc ToExLocE = Import(FromEx.getExpansionLocEnd()))
         MLoc = ToSM.createExpansionLoc(*ToSpLoc, *ToExLocS, *ToExLocE, TokenLen,
                                        FromEx.isExpansionTokenRange());
       else
         return ToExLocE.takeError();
     }
     ToID = ToSM.getFileID(MLoc);
   } else {
-    const SrcMgr::ContentCache *Cache = &FromSLoc.getFile().getContentCache();
+    SrcMgr::FileInfo FromFI = FromSLoc.getFile();
+    const SrcMgr::ContentCache *Cache = &FromFI.getContentCache();
 
     if (!IsBuiltin && !Cache->BufferOverridden) {
       // Include location of this file.
-      ExpectedSLoc ToIncludeLoc = Import(FromSLoc.getFile().getIncludeLoc());
+      ExpectedSLoc ToIncludeLoc = Import(FromFI.getIncludeLoc());
       if (!ToIncludeLoc)
         return ToIncludeLoc.takeError();
 
       // Every FileID that is not the main FileID needs to have a valid include
       // location so that the include chain points to the main FileID. When
       // importing the main FileID (which has no include location), we need to
       // create a fake include location in the main file to keep this property
       // intact.
       SourceLocation ToIncludeLocOrFakeLoc = *ToIncludeLoc;
       if (FromID == FromSM.getMainFileID())
         ToIncludeLocOrFakeLoc = ToSM.getLocForStartOfFile(ToSM.getMainFileID());
 
       if (Cache->OrigEntry && Cache->OrigEntry->getDir()) {
         // FIXME: We probably want to use getVirtualFile(), so we don't hit the
         // disk again
         // FIXME: We definitely want to re-use the existing MemoryBuffer, rather
         // than mmap the files several times.
         auto Entry =
             ToFileManager.getFile(Cache->OrigEntry->getName());
         // FIXME: The filename may be a virtual name that does probably not
         // point to a valid file and we get no Entry here. In this case try with
         // the memory buffer below.
         if (Entry)
           ToID = ToSM.createFileID(*Entry, ToIncludeLocOrFakeLoc,
-                                   FromSLoc.getFile().getFileCharacteristic());
+                                   FromFI.getFileCharacteristic());
       }
     }
 
     if (ToID.isInvalid() || IsBuiltin) {
       // FIXME: We want to re-use the existing MemoryBuffer!
       llvm::Optional<llvm::MemoryBufferRef> FromBuf =
           Cache->getBufferOrNone(FromContext.getDiagnostics(),
                                  FromSM.getFileManager(), SourceLocation{});
       if (!FromBuf)
         return llvm::make_error<ImportError>(ImportError::Unknown);
 
       std::unique_ptr<llvm::MemoryBuffer> ToBuf =
           llvm::MemoryBuffer::getMemBufferCopy(FromBuf->getBuffer(),
                                                FromBuf->getBufferIdentifier());
-      ToID = ToSM.createFileID(std::move(ToBuf),
-                               FromSLoc.getFile().getFileCharacteristic());
+      ToID =
+          ToSM.createFileID(std::move(ToBuf), FromFI.getFileCharacteristic());
     }
   }
 
   assert(ToID.isValid() && "Unexpected invalid fileID was created.");
 
   ImportedFileIDs[FromID] = ToID;
 
   if (FileIDImportHandler)