[NFC] Move FoldingSetNodeIDRef::ComputeHash and FoldingSetNodeID::ComputeHash definitions to header

Lack of ComputeHash inlining slows down slightly FoldingSetBase::FindNodeOrInsertPos calls.
Inlining makes it faster which matters in particular for FoldingSet users in ASTContext.

Extracted from: https://reviews.llvm.org/D118385

Differential Revision: https://reviews.llvm.org/D118612