[CMake] Don't put every project's tblgen executable in the LLVM bin directory.

This changes Clang standalone builds so that clang-tblgen lives in
clang/build/bin instead of llvm/build/bin, and so that with the Xcode
generator it's in clang/build/bin/Debug instead of llvm/build/bin/Debug/Debug/.
Yes, really.

llvm-svn: 197590