[Modules] Make header inclusion order from umbrella dirs deterministic

Sort the headers by name before adding the includes in
collectModuleHeaderIncludes. This makes the include order for building
umbrellas deterministic across different filesystems and also guarantees
that the ASTWriter always dump top headers in the same order.

There's currently no good way to test for this behavior.

rdar://problem/28116411

llvm-svn: 289478