[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

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@176976 91177308-0d34-0410-b5e6-96231b3b80d8

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