Add support for remapping metadata kind IDs when reading in a

Add support for remapping metadata kind IDs when reading in a
bitcode file, so that two bitcode files where the same metadata kind
name happens to have been assigned a different ID can still be
linked together.

Eliminate the restriction that metadata kind IDs can't be 0.

Change MD_dbg from 1 to 0, because we can now, and because it's
less mysterious that way.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@108939 91177308-0d34-0410-b5e6-96231b3b80d8