now that instruction metadata is only parsed in one place, eliminate the

now that instruction metadata is only parsed in one place, eliminate the
parser-global MDsOnInst vector and make ParseInstructionMetadata return
its result by-ref through an argument like the entire rest of the parser.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@92302 91177308-0d34-0410-b5e6-96231b3b80d8