[CMake] Fix linker-as-ld to symlink instead of copy lld

Summary: This test fails if you're building with BUILD_SHARED_LIBS because the LLVM libraries are found via @rpath. Symlinking instead of copying should be sufficiently robust for the test case.

Reviewers: llvm-commits

Subscribers: davide

Differential Revision: https://reviews.llvm.org/D24476

llvm-svn: 281271