[DebugInfo][docs] Fix DISubprogram fields

[ellis]

Summary

D45024 renamed the field in DISubprogram from variables: to
retainedNodes:. Some of the docs were updated in D89082 but this
updates the rest.

[scott.linder]

LGTM, thank you for catching these!

llvm/docs/LangRef.rst

 
 .. _DILocalVariable:
 
 DILocalVariable
 """""""""""""""
 
 ``DILocalVariable`` nodes represent local variables in the source language. If
 the ``arg:`` field is set to non-zero, then this variable is a subprogram
-parameter, and it will be included in the ``variables:`` field of its
+parameter, and it will be included in the ``retainedNodes:`` field of its
 :ref:`DISubprogram`.
 
 .. code-block:: text
 
     !0 = !DILocalVariable(name: "this", arg: 1, scope: !3, file: !2, line: 7,
                           type: !3, flags: DIFlagArtificial)
     !1 = !DILocalVariable(name: "x", arg: 2, scope: !4, file: !2, line: 7,
                           type: !3)

llvm/docs/SourceLevelDebugging.rst

.. code-block:: text
   !llvm.dbg.cu = !{!0}
   !llvm.module.flags = !{!7, !8, !9}
   !llvm.ident = !{!10}
 
   !0 = !DICompileUnit(language: DW_LANG_C99, file: !1, producer: "clang version 3.7.0 (trunk 231150) (llvm/trunk 231154)", isOptimized: false, runtimeVersion: 0, emissionKind: FullDebug, enums: !2, retainedTypes: !2, subprograms: !3, globals: !2, imports: !2)
   !1 = !DIFile(filename: "/dev/stdin", directory: "/Users/dexonsmith/data/llvm/debug-info")
   !2 = !{}
   !3 = !{!4}
-  !4 = distinct !DISubprogram(name: "foo", scope: !1, file: !1, line: 1, type: !5, isLocal: false, isDefinition: true, scopeLine: 1, isOptimized: false, variables: !2)
+  !4 = distinct !DISubprogram(name: "foo", scope: !1, file: !1, line: 1, type: !5, isLocal: false, isDefinition: true, scopeLine: 1, isOptimized: false, retainedNodes: !2)
   !5 = !DISubroutineType(types: !6)
   !6 = !{null}
   !7 = !{i32 2, !"Dwarf Version", i32 2}
   !8 = !{i32 2, !"Debug Info Version", i32 3}
   !9 = !{i32 1, !"PIC Level", i32 2}
   !10 = !{!"clang version 3.7.0 (trunk 231150) (llvm/trunk 231154)"}
   !11 = !DILocalVariable(name: "X", scope: !4, file: !1, line: 2, type: !12)
   !12 = !DIBasicType(name: "int", size: 32, align: 32, encoding: DW_ATE_signed)

 variable ``X``.  The metadata ``!dbg !14`` attached to the intrinsic provides
 scope information for the variable ``X``.
 
 .. code-block:: text
 
   !14 = !DILocation(line: 2, column: 9, scope: !4)
   !4 = distinct !DISubprogram(name: "foo", scope: !1, file: !1, line: 1, type: !5,
                               isLocal: false, isDefinition: true, scopeLine: 1,
-                              isOptimized: false, variables: !2)
+                              isOptimized: false, retainedNodes: !2)
 
 Here ``!14`` is metadata providing `location information
 <LangRef.html#dilocation>`_.  In this example, scope is encoded by ``!4``, a
 `subprogram descriptor <LangRef.html#disubprogram>`_.  This way the location
 information attached to the intrinsics indicates that the variable ``X`` is
 declared at line number 2 at a function level scope in function ``foo``.
 
 Now lets take another example.

 .. code-block:: text
 
   ;;
   ;; Define the anchor for subprograms.
   ;;
   !4 = !DISubprogram(name: "main", scope: !1, file: !1, line: 1, type: !5,
                      isLocal: false, isDefinition: true, scopeLine: 1,
                      flags: DIFlagPrototyped, isOptimized: false,
-                     variables: !2)
+                     retainedNodes: !2)
 
   ;;
   ;; Define the subprogram itself.
   ;;
   define i32 @main(i32 %argc, i8** %argv) !dbg !4 {
   ...
   }