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

[Argyrios Kyrtzidis via cfe-commits]

================
@@ -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(
----------------
akyrtzi wrote:

I think this should work fine for the test case:
```
@@ -496,7 +496,12 @@ bool Scanner::lexModuleDirectiveBody(DirectiveKind Kind, const char *&First,
                                      const char *const End) {
   const char *DirectiveLoc = Input.data() + CurDirToks.front().Offset;
   for (;;) {
+    const char *PriorFirst = First;
     const dependency_directives_scan::Token &Tok = lexToken(First, End);
+    if (Tok.is(tok::hash) && Tok.Flags & clang::Token::StartOfLine) {
+      First = PriorFirst;
+      return false;
+    }
     if (Tok.is(tok::eof))
       return reportError(
           DirectiveLoc,
```

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