[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