[clangd] findNearbyIdentifier(): fix the word search in the token stream.

Without this patch the word occurrence search always returns the first token of the file.
Despite of that, `findNeardyIdentifier()` returns the correct result (but inefficently) until there are several matched tokens with the same value `floor(log2(<token line> - <word line>))` (e.g. several matched tokens on the same line).

Reviewed By: kadircet

Differential Revision: https://reviews.llvm.org/D84912

(cherry picked from commit 05b17346)