[DeclContext] Sort the Decls before adding into DeclContext

Fix a non-deterministic issue in clang module generation, which the
anonymous declaration number from a function context is not
deterministic. This is due to the unstable iteration order for decls in
scope so the order after moving the decls into function decl context is
not deterministic.

From https://reviews.llvm.org/D135118, we can't use a set that preserves
the order without the performance penalty. Fix the issue by sorting the
decls based on raw encoding of their source location.

rdar://104097976

Reviewed By: akyrtzi, vsapsai

Differential Revision: https://reviews.llvm.org/D141625