[clangd] Fix a hover crash on templated spaceship operator.

We make assumption that:
getDeclForComment(getDeclForComment(X)) == getDeclForComment(X)
but this is not true if you have a template
instantionation of a template instantiation, which is the case when, for
example, you have a <=> operator in a templated class.

This fix makes getDeclForComment() call itself recursively to ensure
this property is always true.

Fixes https://github.com/clangd/clangd/issues/901

Differential Revision: https://reviews.llvm.org/D112527