PCH: When writing PCH files, tweak the predicate function deciding whether to...

PCH: When writing PCH files, tweak the predicate function deciding whether to mark a decl as "external" to be closer to reality. 

This still isn't perfect, but I believe it is conservatively accurate at marking decls which IRgen needs to see, while still keeping the "deserialization footprint" on Cocoa.h.

llvm-svn: 82112