[DebugInfo] Print empty MDTuples wrapped in MetadataAsValue inline

This improves the readability of debugging intrinsics. Instead of:

    call void @llvm.dbg.value(metadata !2, ...)
    !2 = !{}

We will see:

    call void @llvm.dbg.value(metadata !{}, ...)
    !2 = !{}

Note that we still get a numbered metadata entry for the node even if it's not
used elsewhere. This is to avoid adding more context to the print functions.

This is already legal IR - LLVM can parse and understand it - so there is no
need to update the parser.

The next patches in this stack will make such empty metadata operands more
common and semantically important.

Related to https://discourse.llvm.org/t/auto-undef-debug-uses-of-a-deleted-value

Reviewed By: StephenTozer

Differential Revision: https://reviews.llvm.org/D140900

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