[Clang-tools-extra] Mark override a function which overrides a virtual one

Function void run() on line 286 overrides a virtual function on line 92 of
clang-tools-extra/clangd/index/dex/dexp/Dexp.cpp.  Not marking it override will
cause a build failure when we use -Werror (every warning is treated as an error).

Reviewed By: kbobyrev (Kirill Bobyrev)

Differential Revision: https://reviews.llvm.org/D84794