[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.

llvm-svn: 143238

clang/lib/Serialization/Module.cpp

@@ -35,6 +35,7 @@ Module::Module(ModuleKind Kind)
     SelectorLookupTableData(0), SelectorLookupTable(0), LocalNumDecls(0),
     DeclOffsets(0), BaseDeclID(0),
     LocalNumCXXBaseSpecifiers(0), CXXBaseSpecifiersOffsets(0),
+    FileSortedDecls(0),
     LocalNumTypes(0), TypeOffsets(0), BaseTypeIndex(0), StatCache(0)
 {}