[clang] set python3 as required build dependency

The required HTMLLogger include file needs python3 to run
resource_bundle.py to bundle all the html/css/js resources. However, if
user sets -DLLVM_INCLUDE_TESTS=OFF, CMake will not find python3 and the
resource bundler will never be executed. This patch set the python3 as a
required build dependency to fix this problem.

Patch By: Avimitin

Differential Revision: https://reviews.llvm.org/D152418