[ASTUnit] Unlike LoadFromCommandLine, LoadFromCompilerInvocation causes a crash if Precompilepreamble

is set to true because there is no FileManager at that point.

Patch by Hurcan Solter!

llvm-svn: 173071

clang/lib/Frontend/ASTUnit.cpp

@@ -1912,6 +1912,8 @@ ASTUnit *ASTUnit::LoadFromCompilerInvocation(CompilerInvocation *CI,
   AST->IncludeBriefCommentsInCodeCompletion
     = IncludeBriefCommentsInCodeCompletion;
   AST->Invocation = CI;
+  AST->FileSystemOpts = CI->getFileSystemOpts();
+  AST->FileMgr = new FileManager(AST->FileSystemOpts);
   AST->UserFilesAreVolatile = UserFilesAreVolatile;
   
   // Recover resources if we crash before exiting this method.