ADT/Matrix: two-dimensional Container with View

Leverage the excellent SmallVector infrastructure to write a
two-dimensional container, along with a View that abstracts out
indexing-arithmetic, eliminating memory operations on the underlying
storage. The immediate applicability of Matrix is to replace uses of the
vector-of-vectors idiom, with one caveat: an upper bound on the number
of columns should be known ahead of time.

Author: artagnon

llvm/include/llvm/ADT/ArrayRef.h

@@ -52,7 +52,7 @@ namespace llvm {
     using size_type = size_t;
     using difference_type = ptrdiff_t;
 
-  private:
+  protected:
     /// The start of the array, in an external buffer.
     const T *Data = nullptr;