[clangd] Move the selection decltype hack to getSourceRange.

Previously, it was in canSafelySkipNode, which is only used to decide
whether we should descend into it and its children, and we still used
the incomplete Decltypeloc.getSourceRange() to claim tokens, which will
cause some tokens were not claimed correctly.

Separate a change of https://reviews.llvm.org/D116536

Reviewed By: sammccall

Differential Revision: https://reviews.llvm.org/D116586