[clangd] Fix division by zero when computing scores

NameMatch could be a float close to zero, in such cases we were
dividing by zero and moreover propogating a "NaN" to clients, which is invalid
per JSON.

This fixes the issue by only using Quality scores whenever the NameMatch is low,
as we do in CodeCompletion ranking.

Fixes https://github.com/clangd/clangd/issues/648.

Differential Revision: https://reviews.llvm.org/D94755