[lld] support --trace-symbol option

I have submitted patch for “–trace-symbol=symbol” (alias -y symbol) option.

This option provides names of all the link time modules which define and reference symbols requested by user. This helps to speed up application development by detecting references causing undefined symbols. It also helps in detecting symbols being resolved to wrong (unintended) definitions in case of applications containing multiple definitions for same symbols with different types, bindings.

Bugzilla - Bug - PR#28226

Patch for review: http://reviews.llvm.org/D21550

Looks like this was submitted as r273536. Thanks so much!

It is awesome seeing a new contributor to LLD!

Thank you!!

-Shridhar Joshi