[llvm] Reland [CGData] llvm-cgdata #89884 (PR #101461)

[Fangrui Song via llvm-commits]

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@@ -0,0 +1,28 @@
+include "llvm/Option/OptParser.td"
+
+class F<string letter, string help> : Flag<["-"], letter>, HelpText<help>;
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MaskRay wrote:

User-facing tools typically use `--foo` (two dashes) style long options. Consider switching to it?

https://github.com/llvm/llvm-project/pull/101461