Rename TargetInfo -> LinkingContext.

Also change some local variable names: "ti" -> "context" and
"_targetInfo" -> "_context".

Differential Revision: http://llvm-reviews.chandlerc.com/D1301

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@187823 91177308-0d34-0410-b5e6-96231b3b80d8