[llvm] [TableGen] Detect invalid -D arguments and fail. (PR #102813)

[Rahul Joshi via llvm-commits]

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@@ -42,6 +42,25 @@ struct {
   { tgtok::Endif, "endif" },
   { tgtok::Define, "define" }
 };
+
+// Returns true if `MacroName` is a valid macro name. Valid macro names match
+// the regular expression [a-zA-Z_][0-9a-zA-Z_]* (see prepLexMacroName).
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jurahul wrote:

I changed the code to have this function usable in both places to we have a single copy. That means most of the rest of the comments do not apply anymore. Can you please take a fresh look?

https://github.com/llvm/llvm-project/pull/102813