Add legacy debuginfo tests

The enum debuginfo patch includes a legacy mode that is used when
building against LLVM 5 and LLVM 6.  The main enum debuginfo tests
have been updated to rely on the new approach and a new-enough gdb.
This patch makes a copy of these tests so that the fallback mode will
continue to be tested.

Note that nil-enum.rs is not copied; it seemed not to provide enough
value to bother.

A new header directive is added, "ignore-llvm-version".  I will send a
patch to update the rustc documentation once this lands.

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