Update `ld.lld --version` string for monorepo.

This patch basically does the same thing as
https://reviews.llvm.org/rL352729 did to clang.

With this patch, lld now prints out a correct version string including
a git commit id like this:

  $ bin/ld.lld --version
  LLD 9.0.0 (https://github.com/llvm/llvm-project.git c027658504fa9e68173f53dedaf223695a65e910) (compatible with GNU linkers)

Fixes https://bugs.llvm.org/show_bug.cgi?id=40780

Differential Revision: https://reviews.llvm.org/D58411

llvm-svn: 354605