Use system clang.cindex by default

The file cindex.py was preventing the configuration of the build when using Clang 15, because this file was changed in the upstream to reflect changes in the headers. Both cindex.py and enumerations.py are shipped with Clang so it is possible to use the correct versions for each Clang version directly instead of relying on a version that shipped with Allen.

This MR uses the OS clang.cindex if a TOOLCHAIN file is not specified. This works as a temporary workaround until we figure out !1136 (closed).