[clang] [Clang] Treat line directives as header names. (PR #211512)

[Yihan Wang via cfe-commits]

================
@@ -1599,6 +1599,21 @@ static bool GetLineValue(Token &DigitTok, unsigned &Val,
   return false;
 }
 
+// Unlike header-names, line directive only support filenames in double quotes
+// but does support empty filenames.
+static void GetFilenameSpellingForLineDirective(const Preprocessor &PP,
----------------
yronglin wrote:

nit: This looks very similer to `Preprocessor::GetIncludeFilenameSpelling`. Do we need to merge it into the existing function? Or rename it to `GetLineDirectiveFilenameSpelling`?

https://github.com/llvm/llvm-project/pull/211512