[docs] Sketch outline for HowToUpdateDebugInfo.rst

Summary:
Sketch the outline for a new document that explains how to update debug
info in various kinds of code transformations.

Some of the guidelines that belong in HowToUpdateDebugInfo.rst were in
SourceLevelDebugging.rst already under the debugify section. It seems
like the distinction between the two docs ought to be that the former is
more prescriptive, while the latter is more descriptive.

To that end I've consolidated the "how to update debug info" guidelines
which were in SourceLevelDebugging.rst into the new doc, along with the
information about using "debugify" to test transformations. Since we've
added a mir-debugify pass, I've described that as well.

Reviewers: aprantl, jmorse, chrisjackson, dsanders

Subscribers: llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D80052
diff --git a/llvm/docs/UserGuides.rst b/llvm/docs/UserGuides.rst
index af0d5ad..0b91b3c 100644
--- a/llvm/docs/UserGuides.rst
+++ b/llvm/docs/UserGuides.rst
@@ -35,6 +35,7 @@
    HowToBuildWithPGO

    HowToCrossCompileBuiltinsOnArm

    HowToCrossCompileLLVM

+   HowToUpdateDebugInfo

    LinkTimeOptimization

    LoopTerminology

    MarkdownQuickstartTemplate

@@ -196,4 +197,4 @@
 

 :doc:`AMDGPUDwarfProposalForHeterogeneousDebugging`

    This document describes a DWARF proposal to support heterogeneous debugging

-   for targets such as the AMDGPU backend.
\ No newline at end of file
+   for targets such as the AMDGPU backend.