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

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@286560 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed