[cmake] Fix host tool path with driver build on Windows (#199152)
On Windows, the llvm-shlib dylib build uses the llvm-nm host tool to
make all symbols visible by default. The LLVM_TOOL_LLVM_DRIVER_BUILD=ON
build would fail because $<TARGET_FILE:llvm-nm> was invalid. This change
passes the name of the symlink / executable copy as a custom property so
things work out and the llvm-nm.exe host tool can be found.