[llvm] [cmake] Skip driver-related code unless LLVM_TOOL_LLVM_DRIVER_BUILD

Disable the code responsible for preparing object libraries
for the driver and filling LLVM_DRIVER_* properties
if LLVM_TOOL_LLVM_DRIVER_BUILD is disabled.  These properties are
consumed only by tools/llvm-driver, and so they are not used at all
if LLVM_TOOL_LLVM_DRIVER_BUILD is not enabled.  At the same time,
the related code breaks standalone clang builds against LLVM built
with LLVM_LINK_LLVM_DYLIB.

Differential Revision: https://reviews.llvm.org/D130158