[scudo][standalone] Introduce the C & C++ wrappers

Summary:
This CL adds C & C++ wrappers and associated tests. Those use default
configurations for a Scudo combined allocator that will likely be
tweaked in the future.

This is the final CL required to have a functional C & C++ allocator
based on Scudo.

The structure I have chosen is to define the core C allocation
primitives in an `.inc` file that can be customized through defines.
This allows to easily have 2 (or more) sets of wrappers backed by
different combined allocators, as demonstrated by the `Bionic`
wrappers: one set for the "default" allocator, one set for the "svelte"
allocator.

Currently all the tests added have been gtests, but I am planning to
add some more lit tests as well.

Reviewers: morehouse, eugenis, vitalybuka, hctim, rengolin

Reviewed By: morehouse

Subscribers: srhines, mgorny, delcypher, jfb, #sanitizers, llvm-commits

Tags: #llvm, #sanitizers

Differential Revision: https://reviews.llvm.org/D63612

llvm-svn: 364332