Add support for auto-generating LIT tests by the build

    
autoconf and CMake flavours both updated to auto-generate files for use
in cpp11-migrate's LIT tests.

Auto-generated files are placed in the build directory under
test/cpp11-migrate/autogen. The directory structure past this point
mirrors the structure in the source directory under test/cpp11-migrate.
A lit.site.cfg is generated in test/cpp11-migrate/autogen to describe
the new test suite. When LIT runs, it runs the standard testsuite and
now also the auto-generated testsuite.

llvm-svn: 175544