build: help pave the road for a mixed Python build

LLDB requires python3, however, gyb is a python2 tool.  Use the fact
that we have a newer CMake at our disposal and use the `Python2` package
in CMake to locate a viable python interpreter.  This allows us to have
references to both versions of python simultaneously.

Author: compnerd

cmake/modules/SwiftHandleGybSources.cmake

@@ -1,6 +1,8 @@
 include(SwiftAddCustomCommandTarget)
 include(SwiftSetIfArchBitness)
 
+find_package(Python2 COMPONENTS Interpreter REQUIRED)
+
 # Create a target to process single gyb source with the 'gyb' tool.
 #
 # handle_gyb_source_single(
@@ -58,7 +60,7 @@ function(handle_gyb_source_single dependency_out_var_name)
       COMMAND
           "${CMAKE_COMMAND}" -E make_directory "${dir}"
       COMMAND
-          "${PYTHON_EXECUTABLE}" "${gyb_tool}" ${SWIFT_GYB_FLAGS} ${GYB_SINGLE_FLAGS} -o "${GYB_SINGLE_OUTPUT}.tmp" "${GYB_SINGLE_SOURCE}"
+          "$<TARGET_FILE:Python2::Interpreter>" "${gyb_tool}" ${SWIFT_GYB_FLAGS} ${GYB_SINGLE_FLAGS} -o "${GYB_SINGLE_OUTPUT}.tmp" "${GYB_SINGLE_SOURCE}"
       COMMAND
           "${CMAKE_COMMAND}" -E copy_if_different "${GYB_SINGLE_OUTPUT}.tmp" "${GYB_SINGLE_OUTPUT}"
       COMMAND