[Modules] Don't eagerly load and associate all the module header files.

In a module-enabled Cocoa PCH file, we spend a lot of time stat'ing the headers
in order to associate the FileEntries with their modules and support implicit
module import.

Use a more lazy scheme by enhancing HeaderInfoTable to store extra info about
the module that a header belongs to, and associate it with its module only when
there is a request for loading the header info for a particular file.

Part of rdar://13391765

llvm-svn: 176976