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

[Rahul Joshi via llvm-commits]

================
@@ -44,6 +44,28 @@ constexpr PreprocessorDir PreprocessorDirs[] = {{tgtok::Ifdef, "ifdef"},
                                                 {tgtok::Endif, "endif"},
                                                 {tgtok::Define, "define"}};
 
+// Returns a pointer past the end of a valid macro name at the start of `Str`.
+// Valid macro names match the regular expression [a-zA-Z_][0-9a-zA-Z_]*.
+static const char *lexMacroName(StringRef Str) {
+  assert(!Str.empty());
----------------
jurahul wrote:

Makes sense. I have tried to reduce the whitespace, and have 3 sections: initial assert, first char check, rest chech. PTAL.

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