CMake: Don't install llvm-tblgen twice

Summary:
The add_tablegen macros defines its own install target, and it was also calling
add_llvm_utility which adds another install target.

Configuring with -DLLVM_TOOLS_INSTALL_DIR set to something other than
'bin' along with -DLLVM_INSTALL_UTILS=ON was causing llvm-tablgen
to be installed to two separate directories.

Reviewers: beanz, hans

Reviewed By: beanz

Subscribers: llvm-commits, mgorny

Differential Revision: https://reviews.llvm.org/D30656

llvm-svn: 297403