[clang] [clang][dep-scan] Resolve lexer crash from a permutation of invalid tokens (PR #142452)

[Cyndy Ishida via cfe-commits]

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@@ -503,6 +503,10 @@ bool Scanner::lexModuleDirectiveBody(DirectiveKind Kind, const char *&First,
           diag::err_dep_source_scanner_missing_semi_after_at_import);
     if (Tok.is(tok::semi))
       break;
+    if (Tok.is(tok::hash) || Tok.is(tok::at))
+      return reportError(
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cyndyishida wrote:

I'm not really sure how to do that without introducing some backtracking. AFAICT, as we consume each token, the `First` keeps moving forward. 

https://github.com/llvm/llvm-project/pull/142452