[clang] [Attributes] Support Attributes being declared as only supporting late parsing when passing an experimental feature flag (PR #88596)

[via cfe-commits]

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@@ -1822,28 +1822,106 @@ void WriteSemanticSpellingSwitch(const std::string &VarName,
   OS << "  }\n";
 }
 
+enum class LateAttrParseKind {
+  LateAttrParsingNever = 0,
+  LateAttrParsingAlways = 1,
+  LateAttrParsingExperimentalOnly = 2
+};
+
+static LateAttrParseKind getLateAttrParseKind(const Record *Attr) {
+  // This function basically does
+  // `Attr->getValueAsDef("LateParsed")->getValueAsInt("Mode")` but does
+  // a bunch of sanity checking to ensure that
+  // `LateAttrParseMode` in `Attr.td` is in sync with the `LateAttrParseKind`
+  // enum in this source file.
+
+  const char *LateParsedStr = "LateParsed";
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Sirraide wrote:

```suggestion
  static constexpr StringRef LateParsedStr = "LateParsed";
```

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