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

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.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@113557 91177308-0d34-0410-b5e6-96231b3b80d8