Allow clients to specify search order of DynamicLibraries.

Summary: Different JITs and other clients of LLVM may have different needs in how symbol resolution should occur.

Reviewers: v.g.vassilev, lhames, karies

Reviewed By: v.g.vassilev

Subscribers: pcanal, llvm-commits

Differential Revision: https://reviews.llvm.org/D33529

llvm-svn: 307849