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