[clang] [clang-tools-extra] [clang] Extend diagnose_if to accept more detailed warning information (PR #70976)

[Erich Keane via cfe-commits]

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@@ -179,13 +180,85 @@ class DiagnosticMapping {
 class DiagnosticIDs : public RefCountedBase<DiagnosticIDs> {
 public:
   /// The level of the diagnostic, after it has been through mapping.
-  enum Level {
-    Ignored, Note, Remark, Warning, Error, Fatal
+  enum Level : uint8_t { Ignored, Note, Remark, Warning, Error, Fatal };
+
+  // Diagnostic classes.
+  enum Class {
+    CLASS_NOTE = 0x01,
+    CLASS_REMARK = 0x02,
+    CLASS_WARNING = 0x03,
+    CLASS_EXTENSION = 0x04,
+    CLASS_ERROR = 0x05
+  };
+
+  static bool IsCustomDiag(diag::kind Diag) {
+    return Diag >= diag::DIAG_UPPER_LIMIT;
+  }
+
+  class CustomDiagDesc {
+    diag::Severity DefaultSeverity : 3;
+    unsigned Class : 3;
+    unsigned ShowInSystemHeader : 1;
+    unsigned ShowInSystemMacro : 1;
+    unsigned HasGroup : 1;
+    diag::Group Group;
+    std::string Description;
+
+    auto get_as_tuple() const {
+      return std::tuple(DefaultSeverity, Class, ShowInSystemHeader,
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erichkeane wrote:

nit: since this is just being used for comparison, would just a `std::tie` here be better?  That way the `string_view` isn't necessary?

https://github.com/llvm/llvm-project/pull/70976