Discard metadata produced by LLVM 2.7. The value enumeration it used

is different from what the code now uses in a two ways: NamedMDNodes
were considered Values and included in the numbering, and the
function-local metadata counter wasn't reset between functions.

The later problem breaks lazy deserialization, so instead of trying
to emulate the old numbering, just drop the old metadata. The only
in-tree use case is debug info with LTO, where the QOI loss is
considered acceptable.

llvm-svn: 113557