IR: Disallow complicated function-local metadata

Disallow complex types of function-local metadata.  The only valid
function-local metadata is an `MDNode` whose sole argument is a
non-metadata function-local value.

Part of PR21532.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@223564 91177308-0d34-0410-b5e6-96231b3b80d8