[scripts] Add script to print clang prebuilts version

Bug: http://b/187231324

This script parses soong/cc/config/global.go to return clang prebuilts
version.  (It may get the version from a json config file in the
future.)

Currently it's used in development/scripts/symbol.py and is the
recommended way to get clang prebuilts version number in the future.

Test: get_clang_version_test.py
Change-Id: Id92a74fd523a5270999bb4d6454525bf2f2f0c04