[llvm] [Docs][CodeView] Document `S_LOCAL` and `S_DEFRANGE*` (PR #190156)

[via llvm-commits]

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@@ -184,27 +184,156 @@ S_ENVBLOCK (0x113d)
 S_LOCAL (0x113e)
 ^^^^^^^^^^^^^^^^
 
+Defines a local variable.
+This record is followed by a series of ``S_DEFRANGE*`` records that define the
+live range of this variable.
+
+All ``S_DEFRANGE*`` records consist of a header followed by
+``LocalVariableAddrRange`` and a list of ``LocalVariableAddrGap`` (until the
+record length is reached).
+
+.. code:: cpp
+
+   /// A live range of a variable.
+   struct LocalVariableAddrRange {
+     /// Starting offset in the section
+     uint32_t OffsetStart;
+     /// Index of the section
+     uint16_t ISectStart;
+     /// Size of the range
+     uint16_t Range;
+   };
+   /// A subrange of a `LocalVariableAddrRange` where a variable is _not_ live.
+   struct LocalVariableAddrGap {
+     /// Start of the range, relative to `LocalVariableAddrRange::OffsetStart`
+     uint16_t GapStartOffset;
+     /// Size of the range
+     uint16_t Range;
+   };
+
+The following records only describe the header.
+
 S_DEFRANGE (0x113f)
 ^^^^^^^^^^^^^^^^^^^
 
+.. code:: cpp
+
+   struct DefrangeSymHeader {
+     /// DIA program to evaluate the value of the symbol
+     uint32_t Program;
+   };
+
 S_DEFRANGE_SUBFIELD (0x1140)
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
+A live range of sub field of variable (e.g. ``local.i``).
+
+.. code:: cpp
+
+   struct DefrangeSubfieldSymHeader {
+     /// DIA program to evaluate the value of the symbol
+     uint32_t Program;
+     /// Offset in parent variable.
+     uint32_t OffsetInParent;
+   };
+
 S_DEFRANGE_REGISTER (0x1141)
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
+A live range of en-registed variable.
----------------
Nerixyz wrote:

Rewrote this to "A live range of a variable living in a register".

https://github.com/llvm/llvm-project/pull/190156