build: Add Windows port of LLVM LTO config; start using thin archives with lld.

This uses "-Xclang -emit-llvm-bc" because the clang-cl driver does not
support LTO.

BUG=507755
R=thakis@chromium.org

Review URL: https://codereview.chromium.org/1432133002

Cr-Commit-Position: refs/heads/master@{#359228}
1 file changed