[clang] [llvm] [StrTable] Switch diag group names to `llvm::StringTable` (PR #123302)

[Chandler Carruth via llvm-commits]

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@@ -51,6 +53,14 @@ class StringTable {
     constexpr Offset() = default;
     constexpr Offset(unsigned Value) : Value(Value) {}
 
+    constexpr bool operator==(const Offset &RHS) const {
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chandlerc wrote:

Doh, done. Forgot to go back to this after the iterator facade wanted a member `==`.

https://github.com/llvm/llvm-project/pull/123302