Properly support LLVM_ENABLE_LLD on Windows

Currently, setting -DLLVM_ENABLE_LLD=ON on windows also requires setting
-DCMAKE_LINKER=lld-link.exe. This is both misleading and redundant.

Fix this by trying to find llvm-link.exe when -DLLVM_ENABLE_LLD=ON is
set and CMAKE_LINKER is not, and aborting otherwise.

Differential Revision: https://reviews.llvm.org/D140534