[Serialization] Stable serialization order for OpenCLTypeExtMap and OpenCLDeclExtMap

Sort the elements of Sema::OpenCLTypeExtMap and Sema::OpenCLDeclExtMap
by TypeIDs and DeclIDs to guarantee a stable serialization order.

Differential Revision: https://reviews.llvm.org/D60835

Reviewed By: Anastasia

Reviewers: Anastasia, lebedev.ri
llvm-svn: 358674