[PCH] Keep track of file-level declarations that are contained by files.

Introduce a FILE_SORTED_DECLS [de]serialization record that contains
a file sorted array of file-level DeclIDs in a PCH/Module.
The rationale is to allow "targeted" deserialization of decls inside
a range of a source file.

Cocoa PCH increased by 0.8%
Difference of creation time for Cocoa PCH is below the noise level.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@143238 91177308-0d34-0410-b5e6-96231b3b80d8