Replace hardcoded comment at 'lit.site.cfg.in'

Replace hardcoded comment at 'lit.site.cfg.in'

At the moment almost every lit.site.cfg.in contains two lines comment:

## Autogenerated by LLVM/Clang configuration.
# Do not edit!

The patch adds variable LIT_SITE_CFG_IN_HEADER, that is replaced from
configure_lit_site_cfg with the note and some useful information.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@266515 91177308-0d34-0410-b5e6-96231b3b80d8