swift234

[cmake] Use LLVM_TOOLS_BINARY_DIR from LLVMConfig.cmake and remove any invocation of llvm-config.

There are a couple of things going on in this patch. Lets consider first the
crosscompiling case.

In the cross compiling case, we were already setting LLVM_TOOLS_BINARY_DIR to
the binary dir of the just compiled LLVM (not the native LLVM/clang). This is
exactly what using LLVMConfig.cmake from that just compiled LLVM will set
LLVM_TOOLS_BINARY_DIR.

In the non-cross compiling case, we originally grabbed LLVM_TOOLS_BINARY_DIR
from llvm-config and then used that value to set the NATIVE llvm/clang paths.

rdar://26154980