[ThinLTO/LTO] Don't link in unneeded metadata

Summary:
Third patch split out from http://reviews.llvm.org/D14752.

Only map in needed DISubroutine metadata (imported or otherwise linked
in functions and other DISubroutine referenced by inlined instructions).
This is supported for ThinLTO, LTO and llvm-link --only-needed, with
associated tests for each one.

Depends on D14838.

Reviewers: dexonsmith, joker.eph

Subscribers: davidxl, llvm-commits, joker.eph

Differential Revision: http://reviews.llvm.org/D14843

llvm-svn: 256003