build: update libc++ as there are some bots with integrated build

Some of the builders are building with C++ in the unified build.  Merge
this without review to try to restore the builder.

libcxx/CMakeLists.txt

@@ -41,11 +41,35 @@ if (CMAKE_SOURCE_DIR STREQUAL CMAKE_CURRENT_SOURCE_DIR OR LIBCXX_STANDALONE_BUIL
 endif()
 
 if (LIBCXX_STANDALONE_BUILD)
-  include(FindPythonInterp)
-  if( NOT PYTHONINTERP_FOUND )
-    message(WARNING "Failed to find python interpreter. "
-                    "The libc++ test suite will be disabled.")
-    set(LLVM_INCLUDE_TESTS OFF)
+  if(CMAKE_VERSION VERSION_LESS 3.12)
+    include(FindPythonInterp)
+    if( NOT PYTHONINTERP_FOUND )
+      if(LIBCXX_ENABLE_STATIC)
+        message(FATAL_ERROR "Python is required for building the static library")
+      endif()
+      message(WARNING "Failed to find python interpreter. "
+                      "The libc++ test suite will be disabled.")
+      set(LLVM_INCLUDE_TESTS OFF)
+    else()
+      add_executable(Python3::Interpreter IMPORTED)
+      set_target_properties(Python3::Interpreter PROPERTIES
+        IMPORTED_LOCATION ${PYTHON_EXECUTABLE})
+    endif()
+  else()
+    find_package(Python3 COMPONENTS Interpreter)
+    if(NOT Python3_Interpreter_FOUND)
+      message(WARNING "Python3 not found, using python2 as a fallback")
+      find_package(Python2 COMPONENTS Interpreter REQUIRED)
+      if(Python2_VERSION VERSION_LESS 2.7)
+        message(SEND_ERROR "Python 2.7 or newer is required")
+      endif()
+
+      # Treat python2 as python3
+      add_executable(Python3::Interpreter IMPORTED)
+      set_target_properties(Python3::Interpreter PROPERTIES
+        IMPORTED_LOCATION ${Python2_EXECUTABLE})
+      set(Python3_EXECUTABLE ${Python2_EXECUTABLE})
+    endif()
   endif()
 endif()
 

libcxx/include/CMakeLists.txt

@@ -187,7 +187,7 @@ if (LIBCXX_NEEDS_SITE_CONFIG)
   # Generate a custom __config header. The new header is created
   # by prepending __config_site to the current __config header.
   add_custom_command(OUTPUT ${LIBCXX_BINARY_DIR}/__generated_config
-    COMMAND ${PYTHON_EXECUTABLE} ${LIBCXX_SOURCE_DIR}/utils/cat_files.py
+    COMMAND ${Python3_EXECUTABLE} ${LIBCXX_SOURCE_DIR}/utils/cat_files.py
       ${LIBCXX_BINARY_DIR}/__config_site
       ${LIBCXX_SOURCE_DIR}/include/__config
       -o ${LIBCXX_BINARY_DIR}/__generated_config

libcxx/src/CMakeLists.txt

@@ -283,7 +283,7 @@ if (LIBCXX_ENABLE_STATIC)
     endif()
     add_custom_command(TARGET cxx_static POST_BUILD
     COMMAND
-      ${PYTHON_EXECUTABLE} ${LIBCXX_SOURCE_DIR}/utils/merge_archives.py
+      ${Python3_EXECUTABLE} ${LIBCXX_SOURCE_DIR}/utils/merge_archives.py
     ARGS
       -o $<TARGET_LINKER_FILE:cxx_static>
       --ar "${CMAKE_AR}"