Fix CMake/MSVC when compiler-rt and llvm are built separately

Summary:
For some reason CMake can't find the `append` macro if LLVM is built separately and imported via `LLVM_CONFIG_PATH`.

Patch by Loo Rong Jie

Reviewers: rnk, vitalybuka

Reviewed By: rnk, vitalybuka

Subscribers: dberris, mgorny, llvm-commits, #sanitizers

Differential Revision: https://reviews.llvm.org/D43458

llvm-svn: 327876