title: How to disable Xcode indexing for certain files or subproject?
body: <p>We have a quite large project, which contains both Objective-C and C++ files. All of the C++ files are used during build, but only some of them are ever opened using Xcode, and there are lots of them that should never be opened at all. But Xcode adds all files' information into the index, as a result our .xcindex file has a size of about 40 GB. Obviously, this is very inconvenient.
  My question is: is there a way to disable indexing for some part of the project? For certain source files, group of files, or whole subprojects.
  I tried setting explicitFileType for some sources to "compiled". It disables indexing for them, but they also won't compile. Build system doesn't know what to do with them ("warning: no rule to process file"), even though they are listed in "Compile Sources" build phase. Adding a custom rule for these source files doesn't work either - CompileC command runs, but it has "?" instead of "c++" argument and all clang command line flags are lost.
  Any suggestions would be much appreciated.</p>