[lld] Add .clang-tidy to customize...

[lld] Add .clang-tidy to customize readability-identifier-naming.{Member,Parameter,Variable}Case => camelBack

lld/.clang-tidy is almost identical to the top-level .clang-tidy, with the aforementioned customization.

Reviewed By: ruiu

Differential Revision: https://reviews.llvm.org/D75809