[clang] [Attributes] Support Attributes being declared as supporting an experimental late parsing mode "extension" (PR #88596)

[Aaron Ballman via cfe-commits]

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@@ -592,6 +592,48 @@ class AttrSubjectMatcherAggregateRule<AttrSubject subject> {
 
 def SubjectMatcherForNamed : AttrSubjectMatcherAggregateRule<Named>;
 
+// Late Attribute parsing mode enum
+class LateAttrParseKind <int val> {
+  int Kind = val;
+}
+
+// Never late parsed
+def LateAttrParseNever : LateAttrParseKind<0>;
+
+// Standard late attribute parsing
+//
+// This is language dependent. For example:
+//
+// * For C++ enables late parsing of a declaration attributes
+// * For C does not enable late parsing of attributes
+//
+def LateAttrParseStandard: LateAttrParseKind<1>;
+
+// Experimental extension to standard late attribute parsing
+//
+// This extension behaves like `LateAttrParseStandard` but allows
+// late parsing attributes in more contexts.
+//
+// In contexts where `LateAttrParseStandard` attributes are late
+// parsed, `LateAttrParseExperimentalExt` attributes will also
+// be late parsed.
+//
+// In contexts that only late parse `LateAttrParseExperimentalExt` attributes
+// (see `LateParsedAttrList::lateAttrParseExperimentalExtOnly()`)
+//
+// * If `-fexperimental-late-parse-attributes`
+//   (`LangOpts.ExperimentalLateParseAttributes`) is enabled the attribute
+//   will be late parsed.
+// * If `-fexperimental-late-parse-attributes`
+//   (`LangOpts.ExperimentalLateParseAttributes`) is disabled the attribute
+//   will **not** be late parsed (i.e parsed immediately).
+//
+// The following contexts are supported:
+//
+// * TODO: Add contexts here when they are implemented.
+//
----------------
AaronBallman wrote:

It would be helpful to clarify whether this applies in all language modes or not given that `LateAttrParseStandard` has special rules.

https://github.com/llvm/llvm-project/pull/88596