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.


git-svn-id: https://llvm.org/svn/llvm-project/clang-tools-extra/trunk@175544 91177308-0d34-0410-b5e6-96231b3b80d8
Adding files left out of commit r175544.



git-svn-id: https://llvm.org/svn/llvm-project/clang-tools-extra/trunk@175545 91177308-0d34-0410-b5e6-96231b3b80d8