llvm-config: Set LinkMode in addition to LinkDyLib when using --ignore-llvm

Summary:
LinkDyLib is only used (before arg processing) to set up the default for
LinkMode. So reset LinkMode as well, and process before --link-shared or
--link-static to allow those flags to continue to override it.

Reviewers: beanz

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D27736

llvm-svn: 289608