[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

Author: petrhosek

clang/CMakeLists.txt

@@ -82,10 +82,10 @@ if(CLANG_BUILT_STANDALONE)
   set( CMAKE_LIBRARY_OUTPUT_DIRECTORY ${CMAKE_BINARY_DIR}/lib${LLVM_LIBDIR_SUFFIX} )
   set( CMAKE_ARCHIVE_OUTPUT_DIRECTORY ${CMAKE_BINARY_DIR}/lib${LLVM_LIBDIR_SUFFIX} )
 
-  if(LLVM_INCLUDE_TESTS)
-    find_package(Python3 ${LLVM_MINIMUM_PYTHON_VERSION} REQUIRED
-      COMPONENTS Interpreter)
+  find_package(Python3 ${LLVM_MINIMUM_PYTHON_VERSION} REQUIRED
+    COMPONENTS Interpreter)
 
+  if(LLVM_INCLUDE_TESTS)
     # Check prebuilt llvm/utils.
     if(EXISTS ${LLVM_TOOLS_BINARY_DIR}/FileCheck${CMAKE_EXECUTABLE_SUFFIX}
         AND EXISTS ${LLVM_TOOLS_BINARY_DIR}/count${CMAKE_EXECUTABLE_SUFFIX}