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.

llvm-svn: 266515