[clang][IFS] Driver pipeline: generate interface stubs after standard pipeline.

Up until now, clang interface stubs has replaced the standard
PP -> C -> BE -> ASM -> LNK pipeline. With this change, it will happen in
conjunction with it. So what when you build your code you will get an
a.out or lib.so as well as an interface stub file.

Example:

clang -shared -o libfoo.so -emit-interface-stubs ...

will generate both a libfoo.so and a libfoo.ifso. The .so file will
contain the code from the standard compilation pipeline and the .ifso
file will contain the ELF stub library.

Differential Revision: https://reviews.llvm.org/D70274