[scudo] Implement stricter separation of C vs C++

[scudo] Implement stricter separation of C vs C++

Summary:
Initially, Scudo had a monolithic design where both C and C++ functions were
living in the same library. This was not necessarily ideal, and with the work
on -fsanitize=scudo, it became more apparent that this needed to change.

We are splitting the new/delete interceptor in their own C++ library. This
allows more flexibility, notably with regard to std::bad_alloc when the work is
done. This also allows us to not link new & delete when using pure C.

Additionally, we add the UBSan runtimes with Scudo, in order to be able to have
a -fsanitize=scudo,undefined in Clang (see work in D39334).

The changes in this patch:
- split the cxx specific code in the scudo cmake file into a new library;
  (remove the spurious foreach loop, that was not necessary)
- add the UBSan runtimes (both C and C++);
- change the test cmake file to allow for specific C & C++ tests;
- make C tests pure C, rename their extension accordingly.

Reviewers: alekseyshl

Reviewed By: alekseyshl

Subscribers: srhines, mgorny, llvm-commits

Differential Revision: https://reviews.llvm.org/D39461

llvm-svn: 317097