[CMake] Avoid clang-tablegen-targets dependency when building sphinx docs (experimental)

Proposal to fix bot
http://lab.llvm.org:8011/builders/lldb-sphinx-docs/builds/1564/steps/cmake-configure/logs/stdio

Patch by: Stefan Gränitz

Differential revision: https://reviews.llvm.org/D59232

llvm-svn: 355887