CMake: make LLVM_OPTIMIZED_TABLEGEN friendly with LLVM_EXTERNAL_CLANG_SOURCE_DIR This is need because of clang-tblgen Differential Revision: https://reviews.llvm.org/D26483 llvm-svn: 286560 GitOrigin-RevId: e8e98dcb74cd6d297c31c023bfacdf7f28cdaabc