swift234

[cmake] Use LLVMConfig.cmake instead of llvm-config to set some variables.

Previously in order to support cross-compiling, we were setting these variables
in build-script. Now that we are using LLVMConfig.cmake, passing in these values
are no longer necessary. As an added benefit, Swift when not cross compiling,
does not need to set these variables anyways.

rdar://26154980