Move most of HeaderSearch initialization to libDriver.

For example, adding the default system include paths in clients is now as
simple as

  InitHeaderSearch init(headers);
  init.AddDefaultSystemIncludePaths(langopts);
  init.Realize();



git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@55174 91177308-0d34-0410-b5e6-96231b3b80d8
3 files changed