Swift doesn't produce its own tablegen and by faking out the LLVM CMake infrastructure into thinking it does all of LLVM's build cross compilation support breaks.
This eliminates us setting an LLVM_ prefixed variable in a situation where there is
a viable alternative.
We want to eliminate as many of these as possible since they can cause conflicts
with variables set in LLVMConfig.cmake.
I also added some convenience tablegen functions in AddSwiftTableGen.cmake and
changed all tblgen users to use that instead.
rdar://26154980
