Custom lit.site.cfg configuration for cpp11-migrate

To afford hand-written tests access to any auto-generated headers, a
lit.site.cfg is now created for cpp11-migrate's test directory providing a new
config.substition. Tests can refer to %gen_root in the LIT script.

Removed use of --param clang_site_config. Not necessary when running
tests using the build system.

llvm-svn: 176184