[gn build] Add all non-test build files for lld

Version.inc.in processing has a potentially interesting part which I've punted
on for now (LLD_REVISION and LLD_REPOSITORY are set to empty strings for now).

lld now builds in the gn build. But no symlinks to it are created yet, so it
can't be meaningfully run yet.

Differential Revision: https://reviews.llvm.org/D55593

llvm-svn: 348945