[llvm] [LangRef] Clarify definition of fragments and debug intrinsics (PR #82019)

[Scott Linder via llvm-commits]

================
@@ -130,6 +130,40 @@ debugging information influences optimization passes then it will be reported
 as a failure.  See :doc:`TestingGuide` for more information on LLVM test
 infrastructure and how to run various tests.
 
+.. _variables_and_variable_fragments:
+
+Variables and Variable Fragments
+================================
+
+Source language variables (or just "variables") are represented by `local
+variable <LangRef.html#dilocalvariable>`_ and `global variable
+<LangRef.html#diglobalvariable>`_ metadata nodes.
+
+When variables are not allocated to contiguous memory or to a single LLVM value
+the metadata must record enough information to describe each "piece" or
----------------
slinder1 wrote:

I agree, the wording I proposed is imprecise/ambiguous; I think I also bled over concepts from the "use fragments for everything" world I was originally thinking in terms of.

I like the framing in terms of the number of expressions for many cases, but we still have cases where we use a single fragment+expression to describe that only one piece of a variable is defined, right? Maybe rather than starting with the "reason" for fragments I can start with just exactly what they are, then in a note go into the various cases they are used and give some examples? 

https://github.com/llvm/llvm-project/pull/82019