swift234

[cmake] When not cross compiling, instead of searching for llvm-tblgen, just use the real native cmake target.

This fixes part of the Xcode build by allowing us to avoid having to use cmake
variables with an unexpanded CMAKE_CFG_INTDIR.

Since those variables are not expanded, they point to a non-existant path like:

.../$(CONFIGURATION)$(...)/lib

So finding tablegen fails. In the cross compile case, we still try to find the
native tablegen.

rdar://26154980