ASTMatchers: Replace some copies of the bound nodes tree builder with moves.

But don't move if all we do is clearing the thing. The move method is too large
to be inlined and performs a ton of unnecessary checking when the RHS is empty.

No functionality change.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@216723 91177308-0d34-0410-b5e6-96231b3b80d8