[clangd] Tweak "provides" hover card when symbols have the same name

Previously for overloaded functions we'd show:
  Provides: foo, bar bar bar bar

The symbol name is duplicated
  ==> only show unique names, since we're not displaying the signature

Commas are missing
  ==> fix the logic which was checking for "last element" by value
      (though after the above fix this bug is dead anyway)

While here, remove a redundant bounds check before take_front().

Differential Revision: https://reviews.llvm.org/D150683