[clangd] FuzzyMatch: forbid tail-tail matches after a miss: [pat] !~ "panther"

Summary:
This is a small code change but vastly reduces noise in code completion results.
The intent of allowing this was to let [sc] ~ "strncpy" and [strcpy] ~ "strncpy"
however the benefits for unsegmented names aren't IMO worth the costs.

Test cases should be representative of the changes here.

Reviewers: ilya-biryukov

Subscribers: ioeric, MaskRay, jkorous, cfe-commits

Differential Revision: https://reviews.llvm.org/D47950

llvm-svn: 334712