[Hexagon] Use composition instead of inheritance from STL types

[Hexagon] Use composition instead of inheritance from STL types

The standard containers are not designed to be inherited from, as
illustrated by the MSVC hacks for NodeOrdering. No functional change
intended.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@242616 91177308-0d34-0410-b5e6-96231b3b80d8