This defines the OutlinedHashTree class.
It contains sequences of stable hash values of instructions that have
been outlined. This OutlinedHashTree can be used to track the outlined
instruction sequences across modules. A trie structure is used in its
implementation, allowing for a compact sharing of common prefixes.
This is a patch for
https://discourse.llvm.org/t/rfc-enhanced-machine-outliner-part-2-thinlto-nolto/78753.
